I've just come across what looks like a pretty nasty problem in SSIS for which there is actually a simple workaround.
It involves trying to open up Visual Studio for Applications (VSA) from the SSIS Designer, specifically from the items that use VSA - the Script Task and the Script Component. The problems I was getting were:
When clicking "Design Script..." in Script Component editor I got a message box:
Title: Microsoft Visual Studio
Message: Cannot show Visual Studio for Applications Editor. Additional Information-->The method or operation is not implemented (Microsoft.DataWarehouse.VsIntegration)
When clicking "Design Script..." in Script Task editor the Script Task editor disappears and I'm returned to control-flow surface.
According to this thread: http://forums.microsoft.com/msdn/ShowPost.aspx?PostID=109643 its caused by having C# projects in the same solution as your SSIS project. And the fix? Reboot my friend...reboot!!! Not a particularly friendly workaround it has to be said but it seems to have done the trick.
Hopefully this bug (for it IS a bug) will be fixed before too long!