This should be possible, but I'm not sure what the procedure is to make this work. I'd like to have a OS X partition, Vista partition and a XP partition, and boot between them.
Install xp into a normal bootcamp partition. Then run a vista upgrade to a new partition. This will give you bootcamp and then a windows boot manager window. My next step is to install Ubuntu using wubi and have a quad boot system.
I followed it but am either stupid or there need to be some changes. First of all the Vista Bootloader seems to sit on the 4th partition, selecting the third in rEFIt just fails. And there were some other parts that didn't go that smooth, but I have the three OSes running on one MacBook atm, so it helped me a lot in the end.
Anyway I wanted to participate in the article discussion but first had to register here and now I am lacking five posts, so maybe someone has some more insight.