I have a Mac Mini Intel running Tiger with XP in a separate partition. The process was straightforward with BootCamp. I believe Vista presents a more difficult process but has been achieved by someone who has posted on the net. At some point it is expected that Apple will upgrade BootCamp to accommodate Vista with the same ease as XP.

I believe running under Leapord will present no different challenge than Tiger.

If I recall correctly the person who posted on the net said he upgraded from XP to Vista and did not install Vista directly. To find websites addressing this issue check out "Vista and Bootcamp" under Google.