The simplest answer is that audio and charging will work just fine but the ability to fully control the iPhone from the radio may be lost in some cases.
Early reports from suppliers testing the iPhone 4 say if your radio or kit bears the Works with iPhone (WWI) logo you most likely get control over the iPhone 4–where the song tag info appears on the radio screen and radio knobs control the iPhone 4.
Also if your radio already offers iPhone video capability, it may be compatible with iPhone 4 control.
The reason is an authentication chip is required for iPhone 4 control and if your radio or kit has the WWI label or is iPhone video capable then it has the chip.
Clarion and Alpine said their radios tested so far seem to work with the iPhone 4, but they are still in the evaluation process. Alpine said, “To this point we have tested some but not all head units and have experienced no problems.”
Jensen (Audiovox) is also still testing. A spokesman said, “We are compatible with the iPhone 4, but Apple has some issues to work out. ”
iSimple (Aamp of America) just released new versions of its GateWay car kit so that it can control an iPhone 4 and is offering an upgrade for existing GateWay kits. Owners of the older kits can get a $30 authentication chip for the update.
Also iSimple just published a list of its current products that work with the iPhone 4:
HubVolt (IS42) and HubVolt FC (IS46) chargers
GateWay with PXAMG IC upgrade
SoundByte (PXDP and related SKUs)
TranzIt (IS77) FM transmitter kit
Note: Owners of an iPhone 3G, 3Gs or iPod touch can also upgrade to the new iOS 4 so they must also pay attention to compatibility issues with their car radio or iPod kit.
Source: CEoutlook and iSimple
Photos: iSimple GateWay and authentication chip add-on for GateWay