Are there no TTS gateway apps for the Sidekick/iPhone? I know there's websites that let you type phone conversations.
(Missed this comment earlier)
There are. I'm actually a customer of Sprint IP Relay and have a personal number set up with them that's linked to my Google Talk account. However, making a relay call is generally massively frustrating for the parties on both sides of the line, so I tend to avoid it.
Seriously, I do understand the point you're making, but keep in mind that not everyone is interested in the phone portion of their smartphone, or able to make effective use of it. :)