Giving credit where it's due. I came across this courtesy of Kevin Rose's blog. It's AT&T Lab's Text-to-Speech Demo. There's a 300 character limit, but it's more than enough for interesting and humorous voicemail messages, as Kevin demonstrates in his post.
You can use different punctuation to modify the tone a bit. My own meager example of what is now my cell phone voicemail message is below:
Secure in the Universe (.wav)