I experienced this same problem on the 19th, from morning until night. It was mostly noticeable when using Maps for navigation; the arrow would go from blue to gray, I'd get frequent "Locating" messages, I'd get "stuck" for a mile or so until it would catch up.
I left my 4S off all night, and now it's accurate and smooth again. No resets of any configuration or data were performed.
The weather was clear yesterday; I think it shouldn't have affected GPS. I didn't seem to be having any phone network problems (Virgin mobile 3G CDMA was working fine). The problem persisted whether I was near Wi-Fi APs or not—a good deal of open country on my drive.
Because I can't observe enough in order to definitively answer the question, I must apologize that this isn't an answer per se, but if anyone does run into this again, try turning your phone off overnight, or perhaps even just waiting 24 hours.