Well, for me i finally tried it again, but i end up with the same Result.
I did boot into the Live CD of Ubuntu 10.04 (also tried native install before) , partitionate the SD Card just fine, then ejecting, remount and extracting the NITDroid Rootfs provided in the how-to.

Then, following all steps in the Discription on the N900, mounting the mmcbl1p2 as /and, and then i want to install the kernel.deb file, but then the terminal says that the filesystem is read only.
If i read correctly, we mounted it earlier with read/write permission with:

mount -o noatime,rw /dev/mmcblk1p2 /and

So, then i run the command to install the nitdroid kernel deb but then the Device says that there is no mounted filesystem oö

I even got the Message that the Filesystem should be read-only, how is that possible?

I am still using the 4GB SanDisk MicroSDHC because i cannot afford to buy another one ;-(

Formatted as following: 2100 MB FAT32 as SD
1600 MB EXT3 as NITDroid