If it's some, but not all, which ones (other than the early GH games which have no singing)?
Yes, as long as you have the latest Rock Band 2 title update -- this installs automatically if you're connected to Xbox Live.
Beatles Rock Band and Guitar Hero: Metallica were the first titles to support the wireless mic, anything released after that should support them out of the box as a matter of course.
Guitar Hero 5 was the first Guitar Hero game with support for the Lips mics. Last I heard Rock Band 2 was getting the update to use Lips mics but I don't remember hearing if it actually came out or not.