I switched from ms to apple with lightroom 4 and it was quick and painless.
You do have to change to a Mac software licence and get new software. If you do not upgrade believe you have to sign agreement to shred your old copies.
If you have read some of the posts you should see there are still kinks being worked out with Mt. Lion (10.8.x) and Adobe and the latest Mac updates.
LR is cross-platform. You can use the same serial, you just need to use the Mac installer, obviously. For specifics on migrating catalogs ask in the LR forum. For PS you simply order the correct upgrade and indicate that you want to switch platforms, as indeed platform switches only are possible for current versions...