I haven't read all of your posts but as far as I can see you have the problems I have thought of before.
I'm going to install Debian (Squeeze) on the Sheevaplug and it only supports installing it on SD (as far as I know).
So my concerns are about the limited read/write cycles of the SD card.
Is the OS booted into the RAM so while the Sheevaplug has power there won't be any (unnecessary) writing/reading cycles?
Does the (access)speed of the SD Card have influence on the overall system speed (like copying files over the network, unpacking rar files on the HDD plugged to the Sheevaplug)?
What about the apt-get update files beeing installed? Are they stored in RAM first before written to disk? I would like to install debian squeeze, but since it is under (heavy) development the files will change several times a week and would crash the SD in no time.
What is the size of Debian with some packages like openssl, libcurl, nscurses, rtorrent, libtorrent, proftpd? Is a 2GB SD card enough or should I head for at least 4GB?
Is there any advantage in the SDHC besides "High(er) Capacity" concerning the Sheevaplug?
Last time I checked, yes install to SD media only easily via Squeeze installer...
There are articles on the PlugComputer.org wiki detailing how you can 'minimize' writes to SD cards.
I find a good performance / price point are Class 6 SD cards. I've seen class 10 cards (things scream) but its over kill.
Remember for most systems, once its booted a majority of ram will be caching files read often, so it'll still run fast.
With Flashybrid and the 'prototype' PlugPBX builds being ran right now, once the system boots the SD card is mounted read-only, and any tasks that need write/update access to files have their directories bind mounted to a ram disk. Works slick. Flashybrid scripts do all the heavy work, setting up the mounts, syncing changes back to SD card on system shutdown (or can be triggered at any time using the fh-sync script provided. They even have a 'mountro' and 'mountrw' script that will toggle the entire SD card to read or read/write mode.
So as a PlugPBX user, users can boot the system iron clad. BUT, they can ssh in, issue 'mountrw' and now update the system, modify any file, do whatever they want and when done, issue a 'mountro' command.
For typical day to day use, login, make changes to say FreePBX which updates a SQL database and then when done issue a 'fh-sync' command which commits via rsync changes in the RAM disk back to the SD memory card. It works slick as hell.
You even define what directories are to be 'aliased' to the RAM disk, again, the flashybrid scripts do all the hard work. Its very slick.
I've detailed it on the PlugPBX forums, come have a read...http://forums.plugpbx.org/index.php?topic=32.0
I have yet to find a better solution than this. Its perfect. The stability of say a router type device, but the ability at the flick of a switch to work with it as a regular system, updating, changing and hacking it - and then locking it back down. No more worries about SD card wear leveling issues. I call it lazy mans embedded system design. I'm really really, Lazy.... but also smart....