NEO979  You don't even have to do that - just write an AppleScript like:

say "this is text" saving as "Macintosh HD:Neat file.aiff"

(No "tell" statement needed - the "say" command is in Standard Additions.)  
(Version 2.2)

NEO979  Sorry, that should've said saving TO, not saving AS.  
(Version 2.2)

SHAWN C. BLOWER  The terminal command converts the text to an mp3 using Apple's text-to-speech? I don't think so.  
(Version 1.7)

ANONYMOUS  But you *can* make an aiff with say, then use iTunes to convert it to an mp3. Example:

say -v -o -f

== whatever voice you enter; if you don't enter a voice, it'll use the voice you've chosen in SysPrefs.

== the name of the aiff you save it as; if you don't enter an output file it's read aloud.

== the file you're having read.

As in say -v Victoria -o test.aiff -f testFile.txt

saves the contents of the file testFile.txt as an aiff named test.aiff, read in the Victoria voice.

Then use iTunes to convert it into an mp3.  
(Version 1.7)

MU ADMIN  Not anymore, no 'a' in the download URL. Fixed.

Thanks for pointing it out.  
(Version 1.4)