I don't have sales numbers, but I'd wager that the percentage of PPC phone users that actually post (or even read) these forums is proportionate to the number of talk radio listeners who actually call in. It's always the same tiny group of hardcore listeners, with the occasional first time caller.

FWIW, at this moment there are 58,199 members on this forum (PDAPhoneHome). That would account for roughly $1.5 million in equipment alone, assuming they're all owners of an average $200-300 smartphone or PDA. I think interest in this type of device will only grow once the iPhone is out, and hopefully we'll see a more user friendly interface as a result.