Hi Coolnews,
This matter has been discussed heavily for the last three month. I realized that you are new to the forum. When you go to your local office with your approval letter, you have two choices.
If your passport is still valid, they could stamp it for you and take your finger print (just one finger). The stamp is valid for one year and its written on it Proceed to I-551 ......., which means this is a temporary evidence of your lawful permanent residency till you get your actual plastic card.
If your passport is not valid, they will give you a card that looks like the I-94 card but with your picture in it and stamped with the same wording mentioned above. The process is really easy and probably takes 15 minutes. The trick is that you have to be very early at your local office or other wise you will stay there forever.
Some officers will ask you to hand your old I-94 which proves your asylum status or your RTD or your latest employment card. It really depends on the officer. Just be prepared and patient.
Good luck.