The problem: Hiding Your hbm.xml files in Visual Studio (or not)
The solution: How to nest NHibernate mapping files in Visual Studio projects
I’m doing this in Visual Studio 2008. I don’t know if this is a “bug” or not, but it sure is annoying. The fix Timo posted does work, but it requires editing your .csproj file by hand which is both tedious and error-prone.