My roommate bucko just taught me this command! It's kind of fun to use in a casual way:

say Moo

But one nifty thing you can do with it, possibly, is have the computer translate a text file into a sound file (say, the first chapter of Alice in Wonderland:

say -o "alice.aiff" -f alice.txt -v "Alex"

It's not the best vocalization, but if you want to have your iPod dictate something to you on the bus, it could be useful. You'll probably want to convert the file to an MP3, though, to save space. The -v option specifies which voice to use -- you can find a useful list someone compiled here or see the list in the Speech Text-To-Voice system preference panel.