Interestingly, I found that when I left my iphone out in the car (for almost 2 days) I never lost data. Brought it in to work and sure enough, later in the day I lost data and I had to do the airplane mode on and off trick. So, I'm thinking that when the phone had very poor reception at one point indoors it disconnected from the data network and didn't get back on properly (even though the "E" symbol was there).
My AT&T 3G iPhone (on AT&T) has worked flawlessly. Makes me sad because I really don't want to switch to AT&T
