Speech synthesis technology has improved considerably in the last few years. The state-of-the-art has moved considerably away from the robotic monotones of yesteryear, and some speech engines are now capable of inflection.

Listen to these examples of accents and dialects using the Rhetorical Systems rVoice speech engine:
Prerecorded
Interactive

Dare to compare! Sixteen speech engines go head-to-head in this extensive comparison:
http://www.tmaa.com/tts/comparison_USEng_highres.htm

Pay particular attention to the samples from Aculab and AT&T, and especially Loquendo and Voiceware.