DISCLAIMER: I have a 100% legal version of Windows XP Pro (given to me directly from a Microsoft rep, BTW), but I choose to use the pirated version because I disagree with product activation in principle.
Onto the real subject....
I currently have the RC1 candidate of SP2 installed on my "less than legal, but not entirely illegal" copy of Windows XP Pro and it works fine. For that matter, SP1 worked perfectly fine also. The "key" is to have a product key other than the FCKGW-...... one that everyone else has.
Moral of the story is, never call something unhackable or uncrackable, because that will just make people more determined to hack it or crack it. Alcatraz was supposed to be unescapable. All it took them was a spoon and some time to get out. All it will take to get around any countermeasures by Microsoft will be a keyboard and some time. Maybe a spoon to eat some ice cream while they hack it, but still nothing too sophisticated.