Here's the latest on MS Vista (USA only).
Stores are offering free upgrades with the purchase of any Win XP Media Center or XP Pro desktop or notebook, between Oct 26, 2006 and March 31, 2007.
After purchase, you have to register with MS for the free upgrade, you'll receive an e-mail confirming and the upgrade will be shipped directly to you.
Here's what is interesting, seems that those in the know are getting the upgrade but not installing it yet. Word is that they are waiting for the patches that MS will most surely put out in March 2007 and then do one installation.
Oh, 1 G Ram recommended.