Although I'd previously noticed the Data Compare and Schema Compare options on the Data menu in Visual Studio, until today I didn't have a good reason to see what they can do.
Note I'm running VS Ultimate; if you can't see it, I'm not certain in which editions of the product this menu makes an appearance. SQL Server Management Studio 2008 doesn't seem to have a comparable tool that I'm aware of—which seems odd—but please comment if you know something I don't.
So here's the situation: two copies of the same database, one that's been moved to a different server and both of which have data changes. Your mission: find and fix the differences.
Open VS, Data –> Data Compare –> New Data Comparison…
Luckily I was able to work around this problem (feature?) by re-opening VS, Data –> Transact-SQL Editor –> New Query Connection, connecting to any ol' database (or optionally just cancelling out of the connection window) and then launching the compare wizard again.
I'm not sure what this does since you specify a database connection in the compare wizard anyway and cancelling the connection window or closing the new query window after opening the Transact-SQL Editor connection makes the problem go away. I've logged a bug with Microsoft connect.
Update 6 Sept 2010: MSFTConnect just got in touch to tell me this is likely related to the Solution Navigator extension in the Productivity Power Tools. Disabling apparently fixes the problem and the problem should be fixed this month.