Hmm... I just tried the procedure to boot from SD card.
I used the maemo wiki:

http://maemo.org/maemowiki/HowTo_EAS..._From_MMC_card

and followed exactly these instructions.

The bootmenu consists of:
boot from flash
mmc
mmc2, partition 2, ext2
...
I get the error : Boot from mmc failed, booting from flash...
when choosing the option "mmc2,partition 2..."

When choosing "mmc" it boots, but boots from flash...
What's wrong?

Some specs:
N800 latest firmware
internal sd card, 2gb
mmcblkp1 ~1.5gb FAT16
mmcblkp2 ~0.5gb ext2
Partitioning done with sfdisk: http://maemo.org/maemowiki/HowTo_EAS..._your_MMC_card

I realized that the instructions from the wiki differ from the one at the beginning of this thread. In this thread used:
What's the meaning of these steps?

EDIT:
While partitioning my SD card with sfdisk it asks me whether I want to mark the Fat16 partition as active or not. I chose yes (to mark as active). Was this decision ok?