In Israel, it was implement in Dec 2, 2007.
now you can keep your own number, and move between carriers.
In Israel we had three major carriers: Cellcom, Orange and Pelephone, to move between Cellcom and Orange all you need to do is get a new SIM card, they both work with GSM, with pelephone you need to change the phone, they do not use SIM card and the Portability is from mobile to mobile or fixed line to fixed line.
How it is in another countries ?