Bought one at the ATT store when I bought the iPhone4. It works fine - but must remove case / bumpers to get the connector to fit. Someone should make an adaptor to get the old connectors to fit the iPhone4 slot.
My Belkin charger does work but I have to remove the bumper for it to seat properly. So basically I remove the bumper when I want to charge in the car or get a new charger....the free enterprise system at its finest. At least my iPhone 4 works 100% perfectly.
Nearly all chargers and open-seat docks work with the i4 in a carryover to older technology. You do have to remove the bumper/case as there is a 2/16th or so of an inch added underneath the i4 that can cause an improper docking.
Form factor docks would not work except through 2G which some have said works. The 3G/3GS form factor can't fit well at all so that hurts most people as car docks, GPS car kits (except by DIY modification like Velcro), and a few other form factors don't connect properly.