I'd like to rewire Windows' default behavior so that clicking on an email address anywhere (browser, document, etc.) opens in Gmail instead of Outlook. Is that possible? (Windows 7)
A full-featured alternative would be to install Affixa (the free version is fine) - it enables all context menu functionality (Send as attachment, Send to Email Recipient), as well as setting Gmail to your default client. I've used it successfully in XP and W7.
This solution also works with Google Apps and Yahoo! Mail.
There's now installable Google Apps software for Windows - it's really just a wrapper for the web app in Chrome but that registers with Windows as an email client that you can then set as your default just like you would with Outlook or Thunderbird. Works like a charm.
http://pack.google.com/intl/en/pack_installer.html (uncheck the stuff you don't need).
Another option is to use a browser plugin or userscript to convert Mailto: links to the format required by Gmail.
Basically Chrome offers an easy way to do this by changing your default email protocol.
How to set Gmail as your default client in Google Chrome
Open your Google Mail in Chrome.
If displayed on the right of the address bar you should see symbol. Click on it.
Select Allow mail.google.com to open all email links. Click Done.