I'll just share a story:

I was convinced that my GPS was broken, because I could only get an AGPS accuracy level fix, and whenever I started GPSJinni or other location aware programs, the little satelite dish icon would appear, stop blinking like it had gotten a lock, but would only tell me my location to an accuracy of about half a mile.

Finally, I went into settings -> location and saw that the "enable" checkbox for GPS was blank. I think I had disabled GPS once, but it didn't turn back on automatically. Hope this might help some of you.