My Samsung Galaxy S doesn't recognize when it is plugged in to USB
This has happened many times to me, and every time it has been a bad cable. If you have one, try finding another USB cable to test it with, and see if it works. Often some connection goes bad in the cable just from use and wear and it will only charge and refuses to transmit data. 

The other possibility (this is known to be a problem with the EVO 4Gs, but i don't think i've heard of it on the Galaxy S) is that the USB port on your phone has been damaged or gone bad. This happened to my dad's phone (EVO) and they sent him a replacement.

Let us know what happens after you try a new cable!