This morning, found this blog post:
Herman Bos » Blog Archive » Got new hardware (DG965WH); yay; doesn’t even boot
Normally Linux doesn’t need to have its partition marked with the boot flag (windows does), so I didn’t really guess something like this myself. But it seems that the Intel BIOS checks the partition table and only agrees to boot from a disk if a partition is marked with the boot flag.
Intel, you utter, utter cocks.
I ran into this recently. Bloody confusing.
I can only begin to dread what would happen if you did not have an MS-DOS disklabel.