Some mfgs used the 'not recommended and now depreciated' 12volt firewire (1394) pins instead of the recommend 5volt USB pins for charging. Hense the reason 2G chargers do not work with the 3G. I've also found video cables and an expensive home radio/charger/speaker that work on the 2G and not work on the 3G. I would be very surprised every 3G accessory works with the 3GS. Just can't trust Apple anymore. Personally, I'm getting tired of replacing my accessories everything time I get a new iPhone. Very easy to double and tripple the true cost of a new iPhone.
Hope your success is better than mine
Just a thought. Maybe a new (maybe sticky) thread titled "3G accessories that work with 3GS" is in order.