As Rob said, I'm using offline created PCM samples, which are generated with AT&T Labs Natural Voices which is the best TTS I've ever heard and which can be freely used.

It can be used freely nn times a day from their web site, but a single user desktop licence (with which the PCM output cannot be distributed) costs $49. More useful commercial licences are somewhat more expensive, but still excellent value for money considering the quality.

Rob