This is something I haven't noticed before but I think it's mighty cool!
When you choose to Include the Actual Execution Plan in SSMS 2008 it suggest any missing indexes it thinks that are needed. Plus it also calculates the performance benefit from adding them.
I'm guessing the number (green 84.7361 in the picture) is in percent... So it's like a Database Tuning Advisor "Lite" :))
And best of all this also works when you're connected to a SQL Server 2000 or 2005.
This is what it looks like:
I'm starting to like this new SSMS more and more.