On which directoy the sdcard is mounted?
can i use ext3 as a file system on the sdcard or would i run into problems?
can i mount the sdcard as an ext3 partition on my linux box?

ccc1

External card is mounted automatically only as vfat at /media/mmc1. Second partition of internal mmc is mounted as ext3 at /home.
It seams that phone doesn't want to work with ext3 card even if it is mounted manually from terminal in the same place (at least the gui apps don't work, but you can use it from terminal). I think it should be marked as bug. Technically there should be no problems using ext3 filesystem on mmc card.
If the card is in ext3 format there is no problem using it in linux.

oh, thats a pity ... so we can't have files bigger than 4gb on the n900 ....

You can post a bug report asking for ext3 support on mmc. As I mentioned, I don't see any technical problems for in support. I will gladly vote for it.

My guess is that Maemo 5 still mounts the card the way it's done on earlier releases, and if so you can fix it.

IIRC on earlier releases you could simply edit /usr/sbin/mmc-mount and change the '-t vfat' as you wish. You would have to become root first, of course.