I thought of doing this before, but couldn't figure out how until I read the hint on Safari and speakable items web browsing. There are a few easy things to do to get your Mail.app to read out your email for you. I find it handy while I'm getting dressed in the morning.
Open your Speakable Items (System Prefs) and turn it on. Switch to your Mail.app and say "Define Keybord Commands." A window will pop up and ask you to enter a key command. Enter "command + a" and in the field for "Speak this text," type in "Select". Do the same thing again but for the key command enter the "tab" key and type in "tab" for the text.
Now go back to your Speech preference pane and under the Spoken User Interface section, check off "Select Text When Key Is Pressed" at the bottom and define the key. I used "command + " because I have no other use for it.
Now switch to the Finder and use the "Define Keyboard Commands" command again. Use the "command + " as the keys and type in "Speak" for the text. Now check off 'Use Command in: Any Application' and you're done. Now when you go to the Mail.app ("Switch to Mail" vocally), when you have mail say "Select" (or "Tab" if Mail is in the uper window, this will take you to the bottom window with the text), then "Speak".
[Editor's note: I have not tested this myself as I'm currently microphone-less...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030122061406405