Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_1 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5F136 Safari/525.20)
I have experienced that with my non jail broken phone. It seemed to help when I did a hard reboot by holding the power/lock button and Home button until you get the Apple screen. I haven't had the issue since I did that.