1.) They only work on the Xbox 360
2.) You can not add them on ("stacking") to each other to build up online time.
3.) You can use multiple 48 hour codes on one account if you enter them after the previous one expired.
4.) You cannot use them on gold accounts, only silver (why would you want to?)
5.) A 48 hour trial doesn't make an account gold, it just allows you to play online.
I hope that helps some of you.......
You can get 48 hour trial codes through the tropicana contest.