Has anyone got a system up that reliably boots from SD, having the whole FS on the SD only? Or USB?
Yes. People have had some success at both. An SDcard can be used for the root filesystem (in lieu of the NAND FS) w/o too much trouble, but a newer Uboot than what was supplied with the Plug is required to boot the kernel off the SDcard. See:http://plugcomputer.org/plugforum/index.php?topic=183.0
Also, for EXT2/3 FSs to support booting a kernel, they must have been created using the "-I 128" option (128 byte I-nodes) per:http://plugcomputer.org/plugforum/index.php?topic=460.msg2861#msg2861
You might also want to take a look at (especially section 8; and BTW, I have no idea why rc3 loads the kernel at 0x400000 rather than the normal 0x800000):http://plugcomputer.org/plugforum/index.php?topic=377.msg2548#msg2548
I am still booting the kernel off NAND, but using an SDcard as the root FS. I haven't looked too seriously at booting off the SDcard, since I didn't create my FS with 128 byte I-nodes, but I agree it would be nice to be able to do so. Let us know if you are successful. Oh, and BTW, the SDcard goes into the slot on the Plug contact-side up. This tripped me (and a couple other folks) up at first.
Does uBoot support automatic recognition of bootable images on SD & USB prior to resorting to internal flash?
No, but someone came up with a neat trick which essentially accomplishes the same thing:http://www.openplug.org/plugwiki/index.php/Multi-Boot
Also, the Marvell team has proposed a new version of Uboot that is capable of checking for and automatically reflashing a new Unix if it finds suitable Unix install software on an attached thumb drive. See:http://plugcomputer.org/plugforum/index.php?topic=120.0
What steps are necessary to get a bootable format on SD or USB that works with Plug?
See above for the SDcard boot solution. For USB devices, it is more problematic. Someone correct me if I am wrong, but I've only heard of people successfully booting from a USB device when it was the only device attached to the USB port (or to some hubs when they are in turn attached to the Plug's USB port). In the latter case, it is possible to plug in additional USB devices after the boot - until the next boot - and of course this is not a problem if the only USB device you intend to use on the Plug is your external disk. I think some hubs might be better supported by Uboot than others. I've never had any success with USB boots with my Plug except when I removed the hub and had a thumb drive plugged directly into the Plug. However, here is a good description of how to go about booting from a USB device if you want to pursue it:http://computingplugs.com/index.php/Booting_entirely_off_an_external_USB_device
Being able to have everything on removable media makes many maintenance issues much easier, as the memory can be plugged into other Linux machines, and any mistakes made can be fixed offline.
Again, I just started tinkering with this, so if this obvious, just give me a link and I go away...
Let us know how your attempts turn out, especially if you are successful!