As Marc shares, a new MSN Search Toolbar has been released that includes tabbed browsing. The implementation is pretty smooth - pretty responsive - but I've noticed a few quirks already. For one, it doesn't seem to work for links that specify a target. For example, all OWA links have target="_blank", which makes it impossible to open in a tab - rather it opens a new window and a new set of tabs. The option to open all MSN Search results in background tabs isn't working for me either... it replaces the current tab instead.
Update: I just realize this is working, but it doesn't do what I expected. When you have a tab open for MSN Search, it opens all subsequent links in a new tab. What it doesn't do, however, is create a new tab when you type a search in the toolbar. The latter would be very useful!
Still, worth a download. The new Windows Desktop Search UI made great strides in the 1.0 version, and I expect similar improvement here as this gets to a wider audience and matures.