Putting Powermats in cars is a great idea. There are two reasons to plug your phone into your car: to get audio out of your phone and to get power into it. With the setup I have for the iPhone, it’s one cord–the proprietary 30-pin connector–so I guess that’s ultimately not a big win. However, one of the drawbacks to the Nexus S’s approach with its Micro-USB connector is that I have to plug two things in when I get in the car: the Micro-USB for audio and the headphone jack for audio. I’m sure someone will (or has, and I don’t know about it) come up with a way to put those two things together in just the Micro-USB connector as they have for the 30-pin iPhone connector, but I haven’t seen it yet.
In any case, making it so you don’t have to plug anything in if you don’t want to is a positive step. Plus, Powermats work on other things besides just phones.