I'm another ext3 user, I'm afraid... (although I'd really prefer to use JFS.
OK. I've reconfigured the kernel to statically link EXT3, EXT4 and JFS (I'm a big fan of JFS myself, I've never had a problem with it). So now you can mount a root with any of those filesystems. I've also added device mapper modules for RAID fans.
To upgrade the plug if you've already installed one of the .30 kernels: delete sheeva-2.6.30-rc4* files and rerun README-2.6.30-rc4.