I'm recently faced with problem that my VS2010 cannot open the database project. It's ends with:
<path>.dbproj : error : Object reference not set to an instance of an object.
It happen with existing projects and new ones as well. I tried to repair VS 2010 together with SP1 but without success. I also tried to change instance of SQL Server in Tools->Database Tools->Data Connections
Recently I've installed VS2008 and I guess this had change something. Do you have any ideas?
Finally I solved that issue :) After I run repair on my default instance of SQL Server 2008 R2 it started to work again.