1. I am using D2Plug in original form (as shipped). I only installed WiFi/BlueTooth package, SSH, and some other applications. Since I got D2Plug at the same time as www.plugcomputer.org/downloads/d2plug/ posted image files and source, I believe it's the same as built-in. I did not try to reinstall rootfs/kernel (waiting for something newer than 0.4 version)

2. It was old plain 640x480 DivX or XviD, MPEG4 Simple Profile. I performed side-by-side comparison between D2Plug and several others media players. I did not say playback was not smooth in time, it was blocky and coarse even for static frame. Since D2Plug's player have to upscale 640x480 to 1920x1080, it did it not good enough. I will try repeat this test on more video samples, but first impression was bad, and I lost an interest in using D2Plug as media player (in current state).

3. I put media files to USB drive. SD card class is important only for very high-bitrate video (>24Mbps), which is obviously not the case. Static picture view should not be affected by read/decoding speed.

4. Blocky means the block structure of DCT-encoded frame is visible (16x16 or 8x8 pixels). The edges of blocks are sharp and visible. It may happen during encoding (when target bitrate is too low), but in that case all players should display the same result (unless they perform extra de-blocking filtering, which is actually part of AVC, or MPEG4 High Profile, but definitely not DivX/XviD). Blocks also may appear when decoder's IDCT is not accurate. A lot of artifacts may appear when upscaling is using very simple filters. In case of original video 1920x1080 upscaling is not performed.

I am waiting for s/w update/upgrade, and will repeat my tests (will try to make snapshots).

What resolution did you try? I tried to compare SD documentary playback (low and medium bitrate), compared Philips DVD/DivX player, Patriot and WD media players - the output is smooth. The output of D2Plug is blocky. Should I tinker with settings? Any hint?

My D2Plug's /var/log/kern.log is also flooded, the same messages, size ~24MB
Thank you for the tip how to stop it.

Update: BBxM now even cheaper ~$125. I didn't have a chance to compare the picture quality side-by-side, but D2Plug player output leaves a great deal to be desired. Looks like they use low quality rescaling filters. The low picture update speed makes me feel Marvell's s/w team do not use h/w acceleration or do not tune up video drivers well enough. The s/w version marked as 0.4, meaning it's still under construction.

What kind of browser do you use? I do not see any errors (FireFox 3.5.9). Did you try to contact them via E-mail or Web page request?

Are you interested in enclosed solution, or open development platform?
I have in possession two D2Plugs ~month, so far no problem with heating or PSU. D2P is more expensive than Sheeva/Guru, but it has more features. I did not hear anything wrong about DreamPlug though.

I checked my U-boot:
"U-Boot 2009.08-dirty (Aug 22 2011 - 22:36:00) Marvell version: 5.4.4 NQ"
It does recognize printenv and other commands, though I did not try all of them. Well, manufacturer test did not pass...

I am new in that too. I guess you are within 30-days of warranty period, so why not to ask GlobalScale about RMA or re-flashing instructions?
I think they prefer to give you step-by-step guide, since anyway they are doing that for every device.

If you want the best performance - then it's better to stick with Marvell's proposed system (proprietary optimized drivers, system patches and etc).
Even at this moment this system (v0.4 by GlabalScale revision system) does not look well-tuned - I expected faster graphic operations, also I doubt that power management works as expected (I could not wake up the system after it went to power-safe mode, so I had to disable it). Switching to another kernel means work in generic mode, much lower performance and a lot of non-functional subsystems. I just need it in order to try new platform USB drivers, which were introduced only in kernels later than 35 version.
BTW, if you want better performance for your applications - I would recommend to use Marvell's cross-compiler from download page, and not to use built-in gcc - at least 4-5% improvement in integer operations, especially when specifying the processor type pj4.

used kernel from GlobalScale? It's the same kernel as in shipped D2Plug product, 2.6.32
I would like to try something higher than 2.6.36, since some features are not available in .32

I got two D2Plugs, and they work a bit differently. I enabled WiFi on both. The first one does have WiFi sign on top of the screen, can recognize available networks SSIDs, but does not connect, probably due to security key issue (did not have enough time to play with). The second one still shows "Wireless Network Disconnected", doesn't recognize SSIDs. In both cases iwconfig show mlan0 (instead of wlan0) working, but not getting IP address from router.
BTW. Did you install "d2plug-wifi-bt.deb" package?

The WiFi/BT package is posted on D2Plug download page. I did not have a chance to try it yet.
You should download it and install by command:
> sudo dpkg -i d2plug-wifi-bt.deb

I am playing with D2Plug right now, and thinking to try newer kernel (h/w comes with Ubuntu and kernel 2.6.32). I heard it's possible, but I may loose some optimized h/w features. Though the current video performance seems not impressive (visually, hint me for benchmarks), the system version is called 0.4, which means it's still under heavy development, and may be improved later.

GlobalScale Technologies has removed original documents regarding D2Plug (where the System-On-Chip name was PXA510) Now Marvell is calling it Armada 510, and the official p/n is 88AP510. You can download the kernel and file system source files, and find the h/w drivers inside. Of course, the video and other h/w drivers are Marvell's proprietary, but some of them are the same for whole GuruPlug family (like USB drivers) which is mostly based on Kirkwood line of processors. Not sure about video, probably it's unique for 510 system.

Why don't you look at http://www.plugcomputer.org/development-kits/features.html ?
OpenRD is outdated product, and almost no one has it in stock. You have a big chance to stuck with 10-15 weeks of waiting (factory lead time).
You may get D2Plug or GuruPlug Display faster.
Other non-Sheeva option is a BeagleBoard (plain or -xM). It costs ~$150, has DVI via HDMI (no HDCP) and available in stock from many suppliers (like Digi-Key)