I have been using my Palm car and wall chargers with the iPhone USB cables for a few weeks now. Nothing has blown up yet, and my new iPhone 4s sat on my desk charging all day.
When the Palm Pre chargers came out, they needed 1.0 amps to run the touchstone. The aftermarket and other phones were usually shipping with 0.8 amp chargers at the time, which didn't work properly with the touchstone. The Palm car charger was the key to mounting a touchstone on your vehicle, as everyone quickly learned. If I recall correctly, Palm connected the two *data* pins together in some way inside their USB chargers, so that the phone could detect the higher 1.0 amp power being available versus the 0.8 amps on a regular USB port.
In any case, I have been charging a demo iPhone 4 and now my new 4s for about three weeks using only the Palm USB chargers with the Apple USB cable, and they haven't exploded or exhibited any signs of a problem. The one thing that I *did* discover today wasn't compatible with the Palm charger was a co-workers iPad. The owner said that the iPad doesn't charge from a PC USB port either, only the Apple chargers. I guess they have another way of detecting 1.0 amp chargers that isn't compatible with the Palm method?
As for using your touchstones with an iPhone 4s, there's a blog post showing some guy's hack showing that it is electrically possible if you Google for it. If I can find a right-angle dock connector and an iPhone accessory case to transplant the touchstone charging coils into, I may wind up trying to make a usable version of his circuit myself one of these days.....