I have an iMac with 10.7.5 running fine with Windows XP installed with Boot Camp that works both as native boot and with Parallels. I want to install 10.8 over 10.7 and I have read that XP cannot be installed from 10.8, but will XP/Boot Camp survive the upgrade?
According to Apple's documentation:
If your Mac has a Boot Camp partition with Windows XP or Windows Vista, you can continue to use your Boot Camp partition, but you can’t upgrade to Boot Camp 4.0.
So it sounds like if you upgrade to Mountain Lion, it won't touch your Boot Camp 3.x installation; you won't get Boot Camp 4.0, but you'll be able to continue using XP.
However, I think if you do a clean install of Mountain Lion, that will install Boot Camp 4.0 by default, which requires Windows 7. So you can only keep XP if you do an upgrade, not a clean install.
It's not exactly the same setup, but I had a 2011 MacBook Air running 10.7 + Windows 7 Boot Camp, and the in-place upgrade to 10.8 didn't break anything.
Once installed, the bootcamp partition and the OSX partition are completely separated from one another. You can do anything you want to either one (even deleting them fully) that wont affect a single thing on the other.