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.
Also to add to Charming Ph....... if you change your plan now to umlimited, BE SURE to have it backdated to cover the overages.