Ok, im sorry, i actually had to register in this forum to make this comment, i normally dont chime in but here's my 2 cents.
People worried about dual booting and viruses cross contaminating OS installs..... have you EVER formatted something via NTFS, and then encrypted it with password protection???? IT'S LIKE FORT KNOX outside the OS itself. Back in the day if you had an encrypted NTFS volume and didnt know the password the only way to gain access was either a linux boot disk that required local access to the hardware itself, or a reformat (which wouldnt have been able to be done from a different dual-booted OS). It's not even VISIBLE half the time. Install XP on an encrypted partition, then boot into another install of XP, you WONT be able to even SEE the partition. Windows isnt perfect, i know that, but one place they really shined is with the NT File System. Sure there are a few small workarounds to get access to the inner workings of an encrypted NTFS volume, but not a single one i am aware of that is able to do it via an OS environment virus. And with that said the worst case scenario is that the dual boot configuration deletes mention of the OSX boot loader, but seeing as i dont know how the OSX boot loader (or whatever the mac equivalent works), i'd assume it was rebuildable anyway like the Microsoft version.
Anyway, if you have reasons of your own to not want to dual boot, that's fine, or why you think im stupid to want a dual boot macbook, that's your own opinion, but if these reasons are based on unsubstantiated fear of a virus infecting a different partition from the XP os..... that's nonsense.
Just my 2 cents.
EDIT: The more i think about it, I suppose some of the fear could come from the fact if someone wanted to share both hard drives in each OS install, so like you had access to all files on the XP volume if you were booted in OSX and vise versa, and i suppose i can concede that point, but heck, if you're worried about that... just keep your files separate, duplicate them, or keep them on an external.