My iPhone charges from the home adapter that came with the phone, of course. I also have a 3rd party USB charger that I bought two or three years ago for my iPod. It came with both a home adapter and car adapter. The car adapter will charge my iPhone but the home adapter won't.
Your home adapter probably uses the old FireWire power pins, which the iPhone 3G can no longer charge from.
There is also a handshake, but that's separate and (as of the 3GS) only displays a "product not designed for iPhone" warning - even if it displays the warning, it still lets you use the device.
