I just deposited today, with a prepaid vanilla mastercard and I had no problems at all. I read in a bunch of other post saying that people had troble with the master card. So it might depend on where you live (i live in Canada
Well the things i did, that might help,
1) I deposited on Full Tilt
2) You have to register you card/activate it on the mastercard website. I tried to deposit before i registered the card and the transaction would not work. I then registered, and the deposit worked really fast. Everything has to be the same as the pokerwebsite as on the mastercard website.
3) Im not sure if it makes any difference, but i deposited $99.99.
Hope it helps a bit.