Still no reply on this? I tried again to see if I can figure out what the heck is happening, but still can't.
This issue appears to be the same as mine: http://www.plugcomputer.org/plugforum/index.php?topic=6068.0
I tried to connect using all the known baud rates, and none of them provided a readable console.
I used the SheevaPlug installer, and the kernel provided with it, if that means anything.
I plan on flashing the most recent kernel to see if that resolves the issue.
Linux seems to boot just fine, as it seems, just the console output doesn't seem to be working.