Why doesn't my iPhone play in my car?

Well without knowing the exact situation, my guess would be that the jack on the Belkin is not fitting properly into the slot on the iPhone. If you'll notice, Apple sells an adapter that allows regular headphone jacks to fit correctly into the iPhone slot...it's sunken in deeper than most other devices, which creates issues with the "L" shaped jacks on many headphones, possibly what you have.

but this belkin product uses the adapter that fits in the bottom of the iPod. It seems to fit perfectly in the iPhone....hmmm

Oh well in that case, things that do not specifically say "Made for iPhone" (or whatever the exact syntax is) are not guaranteed to work with the iPhone. Though the dock connecter on the bottom appears to be the exact same, things do not always work exactly the same.

are there any known adapters like the belkin I described for the iPhone yet? I prefer it to the audio jack because u adjust the sound solely through the cars volume and it automatically pauses when u change to the radio or turn off the car.

Thanks!

I had researched this a little before buying an fm transmitter for myself. I found Kensington brand transmitters worked well, so I got one of those from CompUSA for about $50. It works well as far as fm transmitters go, but the best is to have a car stereo headset with an adapter of course.
Pioneer makes a nice headunit that does BT calls and a iPod cord, but I can't find anyone who has it and an iPhone...

The headsets are hit and miss as well, so personally I may wait a bit before buying one, and make do with the transmitter for now.

i use a carkit (through my tape-deck) from a really old cd player, and the iphone works fine as an ipod that way. I only wish that the phone's audio would transmit as well, but it doesn't so i have to take out the carkit to answer the phone anytime I get a call.