Depends on what you use for an email client. Apple Mail? Thunderbird? Entourage? Other?
Don’t have time to figure out right now… but for Mail (or other scriptable app) I’d look into using either Automator and/or AppleScript – both of which support hotkey mappings – to do something like:
(you hit hotkey and…)
– Mail launches
– New email window pops to the fore
You still have the full-fledged mail client, but at least you’re taken directly to what you want to do… create a new email.
OTOH, if you leave Mail up and running in the background all the time, then you’re never that far away in any case:
– Command-tab to get to the Mail icon
– Command-N to create a new email
Hardly seems to be worth the trouble. But maybe that’s just me.