No, the n900 assumes the microSD is formatted FAT, and will fail to automatically recognize/mount it on boot/insert. (For instance my microSD is ext3, I have to mount it manually in Xterm) If you or someone else compiles the necessary kernel modules, you *may* be able to manually mount the microSD, but this is deeply in "at your own risk" territory.
/dev/mmcblk0p1 /home/user/MyDocs vfat noauto,nodev/noexec,nosuid,noatime,nodiratime,utf8,uid=2999,shortname=mixed,dmask=000,fmask=0133,rodir 0 0
/dev/mmcblk0p1 /home/user/MyDocs ntfs-3g noauto,nodev/noexec,nosuid,noatime,nodiratime,utf8,uid=2999,shortname=mixed,dmask=000,fmask=0133,rodir 0 0