Not sure if anyone has noticed this yet but if you run Visual Studio 2008 with the DBML (not sure about EF yet) under Windows 7 the tables will disappear when you hover over them. Looking for a fix so I thought I'd ask everyone since I've seen it asked in comments but not as a question yet.
EDIT
I've filed a bug with Microsoft. Anyone else who wants to help get it fixed please validate it as a bug.
It's unlikely that there is a published fix available for this. Windows7 is still a non-RTM product and will have bugs with existing applications. Since the issue is more likely to be in Windows than Visual Studio, the fix will almost certainly come from Windows.
Your best bet is to upgrade to the next version of Windows7 when it comes out and hope it permanently fixes the issue.
A short term solution is to disable Aero and see if that helps. I've found that occasionally fixes graphics issues for me.
EDIT
I was able to confirm this exists even on newer builds of Win7. It's almost certainly a bug in Windows vs. Visual Studio. But this bug will get actioned much faster if you file a bug on Visual Studio. I've included the bug link below. I'm going to follow up internally but this is likely to get attention faster if it's filed by a customer.
There's a related bug which pertains to the Entity Framework EDMX Entity Model Designer on Windows 7 RC. So the DBML designer is still broken.