I checked the two travel chargers I have, and just as expected: The one that charges the iPhone 3G has a 2.4v and 2.8v reference voltage applied on the data pins in addition to the 5v charging voltage. The one that charge my iPod but not my iPhone have no voltage on the data pins.
So, I guess the solution is to make a small adapter that gives the required voltage on the data pins: USB in, some resistors and USB out. Like described in the article about the DIY iPhone 3G charger.
That simple adapter will solve the problem of some of the USB chargers that won’t work on the iPhone 3G, but not the problem with the equipment that use the old FireWire method of charging (like the Dension ICElink in my car). For that you need a slightly more advanced adapter.