Originally Posted by psylichon
Avoid USB extension when it comes to anything iPhone-related. Trust me, I'm speaking from experience.
Well, I don't know about the 3GS but I've been using this aforementioned USB extension cable to charge my 3G since I first bought it last year with no problems at all. However, like Europa mentioned, I don't use it to sync the iPhone so I couldn't recommend others to do it either.