Lion comes with Boot Camp 4, which requires Windows 7 for new installations. If you are running Snow Leopard with Boot Camp 3 and Windows XP/Vista, and upgrade to Lion, your Boot Camp will likely continue working… however you should not run the Boot Camp 4 utility or Windows XP/Vista will stop working. Also of note is that some people have reported that their Boot Camp has stopped working after upgrading to Lion. This is likely due to the boot drive getting repartitioned for the Lion Recovery HD, and the Boot Camp partition getting messed up.