I originally used one of the many "clamp" type mounts but then found that RAM make a really dinky little mount especially for the iPhone 4 (and a similar one for the 3). This screws to a standard 1" RAM ball so can be used with a RAM windscreen mount or any of the motorcycle mounts.
In the car the RAM suction cup is massively strong and on the bike the iPhone is so secure I can trust it off road.
For power any of the cigarette lighter/USB adapters you can get for a couple of pounds works with the standard Apple lead. Straight in the cig socket on the car or used with a DIN/cig socket adapter on the bike.
I got RAM mount from Amazon/Mountguys. It comes direct from USA so takes a few days but cheaper than ordering from UK distributers.