Thanks archlinuxARM guys!
Using the generic archlinuxarm arm5 root filesystem I got archlinux running on a Netus Foxboard G20 system (Atmel AT91SAM9G20 based on ARM926EJ-S ™ ARM ® Thumb ® Processor with an internal clock speed of 400MHz).
I had to use the uImage and /lib/modules/* files from the root filesystem provided Netus (and their funky custom bootloader & partitioning scheme). Otherwise it seems to work fine...
As expected, a kernel and modules built with the Netus .config works as well. Next up is a little xenomai action.