Upgrade from Snow Leopard to Mountain Lion - what about Bootcamp??

Nope, the installation process upgrades the new Mac OS and doesn't touch Bootcamp. After upgrading it will still be happily sitting on it's partition waiting for you.

Don't forget to upgrade your Windows 7 drivers via Bootcamp Assistant.

http://support.apple.com/kb/PH10749?viewlocale=en_US&locale=en_US

As with any upgrade I'd make sure to have a working backup before starting just in case!