I got it. Finally.
The root filesystem image that Marvell has posted on their site is corrupt. The stupid thing stole three days of my life while I tried to figure it out.
I got the idea after reading your suggestions over here:http://openplug.org/plugforum/index.php?topic=133.0
I pulled out the original CD, and noticed that the filesystem zip is 132MB, whereas the one on their site is only 538k. That should have been a strong indicator that something was wrong with the website one, but it zipped out to a big file, so I assumed it just had some amazingly good compression.
Anyway, I flashed my plug using the image from the disk, and it works fine. It's currently the same version number as what they have posted online (9.0.5).
So, that's the answer: use the image from the disk, not from the website. Thanks plugcomp for all the help. No thanks to Marvell for wasting our time with corrupt crap left on their site for long periods of time.
If anyone has lost their disk, I'm uploading the zip of the 9.0.5 filesystem right here:http://djpandemonium.com/plug/SheevaPlug_FileSystem.zip