If you are one of the lucky folks making the switch to Mac by way of a gift this year, you might want to check out these two really great resources. First, The Unauthorized Apple Weblog has a great line of posts on switching. They cover everything in their Mac 101 series from Address Book to booting from a USB drive.
Then, of course, Apple has their own resource for switchers, Switch 101. It is a bit more basic, but much more organized the TUAW’s Mac 101 series. Finally, don’t forget that you can get your data migrated over from a PC with the help of a genius at an Apple Store Genius Bar… just remember to make an appointment before you go.
Finally, when all else fails, hit Google. The venerable search behemoth can probably point you in the right direction, especially if you use site focused searching. For example, don’t just hammer search terms in, but search a specific site (or even subsite) like this:
site:http://www.apple.com/support/ itunes deauthorize
Typing the above will pull all references out of the support pages at Apple on deauthorizing computers from iTunes.