i had osx running alongside xp/vista on one disk.
at first i installed it on an external usb disk (for easy testing)
and moved the partition later on the internal disk/partition.
to get it booting (as the bios boot selection dosnt work with partitions)
i used easybcd to add the partition to the (vista)bootloader.

i have no experience of how to install directly on the partition, but adding to
the bootloader should work.

btw:
the order bootloaders work should be as follows (correct me if wrong):
1. bios bootloader / preference - which disk
2. loader on first partition on that disk (in most cases win mbr) - lets choose specific partitions
(even on other disks)
3. local loader on that partition - may be the osx darwin bootloader

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Leopard 10.6.5 | Core i3 | Gigabyte H55M-USB3 | ATI 5770 | Coolermaster Cosmos