A good rule of thumb is that any USB-based charging solution that claimed to work with Blackberries should work with the 3G iPhone. Since both devices require negotiation to enter high-power charging mode, any charger that supplies the right voltage on the data lines should suffice.

FWIW, I built my own "universal" cable. It's a USB extension cable cut down to 6" or so, with a couple of resistors and some perfboard in the middle to get voltage on the data lines. That way, I can work with any generic USB port.

I also modded a 12V -> USB cig lighter adapter to put the right voltage on as a universal charger.