I tried again 2day in Terminal on my Mac which SSH connected to N900, and give me feedback like this:

Nokia-N900:~# unmount /dev/mmcblk1p1
-sh: unmount: not found
Nokia-N900:~# sfdisk -uM /dev/mmcblk1 << EOF
> ,<100>,C
> ,,L
> ,,
> ,,
> EOF
Checking that no-one is using this disk right now ...
BLKRRPART: Device or resource busy

This disk is currently in use - repartitioning is probably a bad idea.
Umount all file systems, and swapoff all swap partitions on this disk.
Use the --no-reread flag to suppress this check.
Use the --force flag to overrule all checks.

Nokia-N900:~# unmount /dev/mmcblk1p1
-sh: unmount: not found
Nokia-N900:~# mkfs.vfat /dev/mmcblk1p1 -n sdcard
mkfs.vfat 3.0.1 (23 Nov 2008)
mkfs.vfat: /dev/mmcblk1p1 contains a mounted file system.
Nokia-N900:~# mke2fs -j -m0 -L NITDroid /dev/mmcblk1p2
mke2fs 1.41.3.maemo0 (12-Oct-2008)
Could not stat /dev/mmcblk1p2 --- No such file or directory

The device apparently does not exist; did you specify it correctly?