Yes, better we have dogfights: http://www.history.com/minisites/dogfights/
IMO, an excellent series.
BTT: I think that it's a good idea to have other boot partitions offline or inaccessible. It prevents users from getting in trouble running a scan in XP, for example, that sees system files on the Vista partition that it doesn't recognize. These might be considered corrupt or a heuristic might decide it's potential malware. In my idea of an ideal setup, each boot partition should only contain system files and other OS-specific files. Then there would be no issue with only one being active at a time and the others being offline. In the real world, that's not always practical.
If you must, yes you can assign a letter to the other boot partition. It works, I've tried it. In the last post above from madmacs, the Vista partition has no volume letter assigned when Win7 is booted. That Vista partition can be assigned a letter and accessed, if need be.