I can't edit a stored proc that uses MERGE when using EAP3 against a SQL 2008 database using the SQL 2008R2 SSMS. I have to disconnect my query window in order to use SSMS without throwing a SQL Prompt error. If I'm really quick, I can quickly alter something and then close the window or delete the text, but otherwise, I continually get errors while that code is on-screen.
Not sure what you need in order to repro. I can probably send it via PM if needed.