this happened to me also, a couple of days ago. i had gingerbread running (i don't tether, and if i wanted to i would pay for it). It ran great all day after loading gingerbread, but later that evening, i lost all data connection. The phone still worked but no data connection and no 3G symbol, only the bars.
I tried *228s, battery pulls, cache clears, factory resets and even SBF'd back to .340 - twice - and still no data connection love. I figured I would have to go bring it into a store the following day. So I was sort of looking it over, making sure it was ready to be returned and, all of a sudden, the 3G symbol just popped on. And data connection was restored.
I put gingerbread back on, i don't use tether at all, and haven't had a problem for the past 2 days now.