You basically need to:
1. jailbreak using limerain or pwnagetool.
2. make sure your number is put correctly on settings->phone->my number (ex: +20101234567).
3. add the source http://dl.dropbox.com/u/13972199
4. install the package factime switch. you can search for com.darkvoid and this will get u there.
5. after the device restarts, make sure your wifi is on and then set settings->phone->facetime as on.
That's it. You are good to facetime.
UPDATE 1: iPhoneIslam has released a safer (according to their website) patch which can be found by adding their source http://apps.iphoneislam.com/ and then searching for the application Facetime Hacktivator. Full steps can be found at http://www.iphoneislam.com/?p=6920
UPDATE 2: Bigboss has another patch on their repository with name MiddleWhat