I've tried GUID and MBR and experienced the same problem. Here is my partitions:
Master (120GB IDE):
Leopard
Slave (80GB IDE):
Ubuntu (root, home, swap)
I've tried booting from Slave (grub) and from master (Darwin bootloader). Both get to the darwin bootloader and start to load, but then the computer restarts.
My specs:
P4 (3.0ghz)
1GB RAM
I've installed w/ proper sse2 patches and the correct bootloader/partitions. This is probably about my sixth install this weekend and I'd really like to get it working.