Oy, I couldn’t believe it.GMail now supports setting the from field in your emails to whatever you want:
Gmail: Help Center

I was thinking of hacking this via Greasemonkey but now I guess I don’t have to. And you can even set different emails and then choose when you’re sending what from address you want. Freaking cool! Now I can finally send email from my nice domain name, but still use gmail as my client!

Thanks Google!