I’ve been reading the code commits for the WebKit project and they hint at two things: tabs in Mail.app and Safari and Mail.app will merge.
You heard it here first, folks.
Update: I’m trying to find the notes for the code commits that put this together.
Steve Kirks 
Where are the specific comments? Got any links for me to peruse?
I’ll run through the code commits and put up another post. They’ve also made progress on the Windows port of WebKit and the source code tree now inlcudes a VS project file for a basic browser.