I've been looking for a good API for this, too! I actually just spoke to someone at RIM about this yesterday.
The RTTS key is accessible internally to RIM only. The will not allow public use of it. He stated that if my app needed to be signed by the RTTS key, then I could never run it on an actual device.
Supposedly, tho, they have public "versions" of their TTS apis that they can give out in "some cases". I asked what those were but have not received any reply. I'll let you guys know of anything else I find out.
iSpeech has a free & a paid api available to use, but it's a web accessible api, wherein you have to send a request over the internet in order to translate it to speech. (http://www.ispeech.org/API) So... that's not usable for my purposes, but one of you might still find it handy.
