In addition, http://maemo.org/maemo_release_docum....x/node12.html says that /mnt/initfs/lib/modules has the kernel modules.
In /mnt/initfs/lib/modules/2.6.21-omap1, the following filesystem modules are present:
ext2.ko
ext3.ko
gadgetfs.ko (?)

It's of course possible to install and use other filesystems (Microsofts SMB, for instance) with the right packages, but this is (I think) what ships.