apperantly it looses mmc sometimes but today it did see the sdcards again.
I was able to start an sdcard containing meego (but installed a dev version so only got a xterm).

But starting an sdcard containing nitdroid failed. It starts the kernel, but then says it cannot find any filesystem for root. It shows a list of tried filesystem (types) but the list is emty. Apperantly it needs some sort of initrd image with filesystem drivers. Also tried flasher-3.5 -l -b to boot the nitdroid kernel passing the bootargs like defined in the multiboot nitdroid entry but this also failed with the same message.
Wonder how multiboot does this....