When you look online at data used it is usually 24-36 hours behind. it is not current......
When you have WiFi marked, if you leave the connection it will fall back to a 3G conenction.
On a Droid, unless you are diligent on confirming your connection you will easily go over the 150MB limit. But it can be done.
My cycle starts on the 14th and even though I use my WiFi connection at home all the time, I am aleady at 95MB of data use for the month.
It can take a bit of time to update, like the last poster wrote. If I can make a suggestion, though: with using a droid device, consider placing the unlimited package and cutting the loss from the increase. If you go over, even by a byte, it will charge you for another full 150MB allowance, basically eliminating the savings. The Droid devices are insatiable data hogs (it is like slop to them). I see a lot of instances where users go over this feature when attempting to save a bit of money. On the Droid phones, it is usually not worth the risk of overage.