I just downloaded and installed the arm-2009q3-67-arm-none-linux-gnuabi compiler from CodeSourcery (I'm doing everything on a Gentoo Linux box).
Download your kernel of choice from kernel.org
download the matching patches and config file from sheeva.with-linux.
Apply the patches.
Install the modified setup file - either modify it yourself by looking at the RD one or download the one superpat supplied here http://plugcomputer.org/plugforum/index.php?topic=927.15
Then copy the .config file from sheeva.with-linux into the linux directory and compile using something like
make -j2 ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi- uImage
If you need other changes then obviously you need to do make menuconfig etc.. At least that's all you should need to do, I still have no idea why my initial efforts went wrong unless it's part of the problems with the crypto system and a compiler bug that has been discussed elsewhere.
Since I only needed the modified setup code - the sheeva.with-linux config has all the other options you need for port multiplier and ESATA enabled already - I am still using the modules from sheeva.with-linux but you can compile your own set using
make -j2 ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi- INSTALL_MOD_PATH=../modules modules_install
These assume you have a dual core machine (the -j2)
Then install using the instructions that are around. In my case installing to a SD card it's actually trivially easy to put the card in a SD reader on my linux box and copy the files - also easier to recover if the kernel doesn't work.
For what it's worth the dual 3.5 drive Startech box I'm using uses a Silicon Image 5744 chip - I just opened it up to extract 5V to eliminate the Sheevaplug mains supply. Seems like a reasonably constructed box with an extruded aluminium case although it's not what you need if you want a really small 2.5 inch solution or more than 2 disks and a temperature controlled fan rather than an on/off switch would be nice. I don't know if the cheaper box they do without the removable disk caddies uses the same chip. I haven't tested hot-plugging either as it's not what I need for a home server but the chip spec sheet says it does.
I also used the uBoot from http://www.naobsd.org/sheeva/u-boot-rd88f6281Sheevaplug_400db_nand.bin
but I think this is optional unless you want to boot from a ESATA disk. This version has both SATA and SD card support, the RDbase version doesn't support SD card booting.
With the availability of ESATA enabled plugs it's a pity you can't get official versions of this sort of thing yet and all the official stuff is well out of date. You need a 2.6.30 or later kernel to get port multiplier support in the kernel for example unless someone backported stuff. It's not to me yet clear what kernel actually ships with the ESATA enabled plugs.