3GS iPhone will not update to iOS 5 According the iOS 5 features page on Apple's web site, the 3GS is supported. When I push the "Check for Update" button in iTunes, it says "This version of the iPhone software (4.2.1) is the current version." I'm running iTunes 10.5.142 on Windows. I have also used this same setup to upgrade my iPhone 4.

Is there something else I need to do here? Could it be refusing because this phone does not have a carrier anymore and I use it only over WiFi?
Simple answer: You don't have a 3GS, but rather a 3G. The last iOS update for the 3G was 4.2.1. A 3GS ran all the way up to 4.3.5, which was the last public release of iOS 4. If you are running 4.2.1 and iTunes doesn't even see the 4.3.5 update, then, well, that's pretty telling that you don't actually have a 3GS but a 3G.