I just picked this up from Channel 9
It is planned to include support for opening links in a new tab in an existing browser session.
This is planned to be in the Beta 2 release and comes from Bruce Morgan who leads the dev team for IE (and other stuff).
This means that clicking on a link in an email will open a new tab not a new instance of IE.
This is great as it was a feature that was bugging me and made me revert to Firefox. I will give IE 7 another go at Beta 2 time.
Oh and Bruce points to the preview of Beta 2 which is available.