My workflow is very email-centric and I've often wished I could start a new email in Mail by pressing a keyboard combo in any program. I use QuickSilver, and the regular way of searching for a contact's email address and then creating a new mail that way seems awkward to me compared to the email address completion Mail.app offers.
After many tries, I finally found out how to get Mail.app to open a new message window using AppleScript:
tell application "Mail"
set t to make new outgoing message
set visible of t to true
activate
end tell
Save this as an AppleScript using AppleScript Editor, and then tie a keyboard shortcut to that script using your favorite keyboard shortcut creation tool. Tested with 10.6.1.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091028101802295