The best program I have found for a text-to-speach program is Gespeaker. From the Ubuntu Software Center install gespeaker-mbrola-en. This package will install both Gespeaker and mbrola with English language voices.
Once the package has installed, open up Gespeaker. In Ubuntu 10.04 it will be found under Applications, Sound & Video. Once it is opened you should change the Language setting to: english-mb-en1. It is the best sounding, at least to my ears. The Pitch should be 50, Volume 100, speed 160, and Delay 0. You can adjust these yourself.
To play a text either copy and paste in the window, or you can click Open and select a text file (.txt). Then just press Play. The program allows you to save the audio in a wav file. It also allows you to save the text you typed into the window. There is even a plugin to use it with Empathy or Pidgin. It is a great little program.
Links
https://help.ubuntu.com/community/TextToSpeech
http://code.google.com/p/gespeaker/
