Check out http://neospeech.com/Default.aspx and you can sound like the new Stephen Hawkins! They give you a choice of mens and womens voices in English, Chinese, Korean, and Japanese. The voices will read English text with varying amounts of a native accent.

It's pretty funny.... Just so people might guess it is a computer, I put some stuttering in the voice with the following message:

Thank you for calling me. Please leave your naaaame and telephone number after the tone. If you know my email address, trying sending me an email. Hope to talk to you soon soon soon soon soon

You can forward the wave file player a little bit to get past some introductory text that is added to the front of the message.....

You might have to try a couple of times to get the recording to sound good on your telephone. Try different distances from your speakers.

Sprint PCS Voicemail allows you to record a main message, and also a vacation greeting message. So you can have two variations to switch between.