I followed the guide from Daniel Celeste’s blog (archive link) but ran into a few gotchas along the way that I didn’t see mentioned outside of random forum posts.

Things to watch out for:

• The iMessage Activator website sends it from the 587 area code; if you see a number from 587 that parses correctly and throws a “Response cookie XXX doesn’t match” error, wait until you get another message from Apple and punch that SMS message into the iMessage Activator site. Punching the hex from the iMessage Activator site won’t work, because the authentication token (“cookie”) still won’t match. Only put in hex dumps from the +44 number.
• Make sure you activate from Apple within the time span that the phone is waiting for the activation message.
• In my experience, by the time it says “FaceTime/iMessage activation failed,” it’s no longer listening for messages even if you get them from the log and send them from the iMessage Activator.
• If you can’t get the SMS from Apple between the time you flip iMessage on and the timeout, you need to try again later when the Apple servers aren’t quite so busy.