Since it uses the same dock connector as the iPod, it is at least possible. It will be interesting to see if it "talks" to peripherals the same way as the iPod does. If so, it should work with all iPod accessories.
IMHO, it won't though. The iPhone is based on OSX while the iPod is not. This makes me think that they will communicated differently. If this is the case, maybe some manufacturers will offer firmware upgrades to make their products iPhone compatible.
If you have a line in on your car stereo, you will be able to connect to the iPhone's headphone jack and listen to your tunes through your car's speakers that way. This method won't let you control the iPhone through your car stereo's controls though.