I hear ya, and its very annoying. Im sure the iphone could charge, but I think they blocked the ability in software ( after all, the iphone is seeing power on those pins ).
I think the old way of charging was done on the 'firewire pins', we now use usb and the power ground i believe are on different pins. An adapter could probably be made. Im not sure why they did this, other then to get people to buy all new accessories for nothing.
I had to buy a new car charger, not that big of a deal, but if I had bought at car that has charge/play capiblitites that worked fine with my first iphone and my new iphone didnt work, I'd be furious. Same goes for speakers as well, it seems alot of them will be no longer be of use to the 3g.
Im also sure the reason we are not hearing the GSM buzz on the spearkers is that you are in 3g, I used to get it all the time on the old iPhone in the car, now I only get it when my 3g coverage drops and I goto edge (to me this was worth the $10 extra a month alone

)
There is Digg article that should be dugg if this affects you, I believe this issue is little known to most... it something that should be addressed since most of us have been investing in ipod/iphone accessories for a while now.... i'd hate to be the guy who bought the Bose speaker system for $300 last year only to know find it a paper weight since the software in the phone says its no longer compatible.
http://digg.com/apple/You_can_t_charge_the_iPhone_3G_on_iPod_Audio_Systems