According to them it's not supposed to happen... but for whatever reason I have been for the past 5 months or so, ever since I started the service. Someone on this board thinks it's a legal thing, but whatever the reason I always make sure to fill the day before. I had originally planned to switch to the $45 plan but haven't had to yet. This is what my account reads right now..
Minutes 1368.0 Minutes
SMS/ MMS 3123 Text Messages
Data 78.4 Mb of Web Browsing
Service End Date 03/12/2010
Oh and I use the cards only... I know of people who switched to auto refill and lost the rollover.