Fix for kernel was submited by Juergen Beisert in August, 2009 and can be found here.
Patches for haret (with workaround and proper rx1950 detection) can be found here.
Update: I've submited patch that adds configurable kernel offset to haret maillist. It seems that mainline haret will be able to boot linux on rx1950 soon :)
2nd update: My patches were included into mainline haret. Neat :)