My fiance and I were having the same issue this week. None of her texts were going through, but we thought it was both phones (since she's the only one I constantly text). We're both on ATT, I have a 3G iPhone, she has a BB Curve. Yesterday afternoon we both cleaned our phones w/ compressed air and ejected SIM cards to clean off.
Low and behold, it turns out it was simple as that, just needed a cleaning. 🙂
If nothing else, I would check w/ a number of your friends to see if they've received texts from you, or see if you can get any from them. If worse comes to worse, you may need to call the carrier and see if they can hook you up w/ a new SIM card.
Good luck!