Raise your hand if you use Gmail's (or another web-based email service's) web interface for email correspondence. My hand is up too. In doing so, one thing is is a little annoying is when you click on an email address on a website somewhere, and Apple Mail opens up. If you use another stand-alone app (Entouage, for example) to check your email, it's easy to switch the default app, but if you are all web-based, they don't make it quite so easy.
The following will not open up a Gmail window, but at least it will stop Apple Mail from opening.