Beware of unit tests!!
We have a relatively big solution. In fact, it got so big we had to split it, so now we have 3 relatively big solutions.
Since the only way to handle such big solutions is through careful regression testing we've been using unit tests for quite some time now.
Suddenly, one day, map projects stopped compiling - we would get an error that it cannot find the schema.
Strange, we thought, as we opened the map file to re-select the schema, but even stranger - as expected we'd get the dialog saying the schema could not be found and asked whether we would like to select a new one - we answer yes and Visual Studio tries to open the BizTalk type picker to allow us to select the schema, but that fails to open and from now on every single operation in VS will cause an error message. The only remedy is to close and re-open it.
We tried everything, new schemas project, new maps projects, nothing helped.
The only time the type picker would open is when no references exist in the project (other then the default ones).
That's until Ben here took initiative and looked at it more carefully.
What he found out amazed me - it is all because we had unit tests projects in our solution. As soon as we removed those from the solution it all went back to normal.