boot0: MBR boot0: Done Non-system disk press any key to reboot

Right. I have windows on HDD1. I have OSX on HDD2. They are on different HDDs. I was messing around with HDD2's partitions through linux and then through OSX, everything turned out fine in the end. A couple of days later, I get a kernel panic (probably my first) and I reboot.

boot0: MBR
boot0: done
Non-system disk press any key to reboot

This is what solved my problem:
1. Booted Ubuntu 9.04 live cd
2. Started Partition Editor (GParted)
3. Set my OSX partition's BOOT flag to ON.
4. Reboot

Et voila!

Not this might not work, or might mess things up for you since the same error might mean many, many other things than just your partition not being active.