The white Belkin I'm using with the iPhone that works is at least 3 years old, maybe 4. I purchased this shortly after it was released for the line out feature. I LOVE this thing. I get zero ground noise, using either a cassette adapter, or now my new stereo has an aux port on the front. I use the Belkin line out, directly to the aux, and get amazing sound in the car.
The trick is on the old white unit, is to make sure the music is playing, then plug it in. The message will come up saying it's not compatible, I select NO to switching into Airplane mode, and it works brilliantly with the iPhone. I wish I knew if it was the connector that stops the black model from working, or the plug electronics. My guess is the connector.
My wife has an old white Belkin too, and the plug is in worse shape then mine, so I bought her a new black one too, thinking it would work... grrr... I'm going to try her white one tomorrow and see if it works like mine does.
There HAS to be a simple answer to this, since some work and some don't.