While it is possible, it is a pain in the ass. I'm working on a project now that doesn't use NHibernate (yes, I am surprised too).
There is very simple data acess there, and nearly nothing to do with touching the data, so I thought that I could simply use ADO.Net. It works, sort of, but my code is full of strings, and it is nearly untestable. The main issue is that all the *DataAdapter expect their own *Command, *Connection, etc. Which mean that I can't mock much of it.
Check out this little piece of a test (part of about 50 lines of setup, that leads to two lines of real test.
MockRepository mocks = new MockRepository();
IDbConnectionProvider connectionProvider = mocks.CreateMock<IDbConnectionProvider>();
IDbConnection connection = mocks.CreateMock<IDbConnection>();
IDbCommand getRecordsCommand = mocks.CreateMock<IDbCommand>();
ISender sender = mocks.CreateMock<ISender>();
IDbCommand deleteItems = mocks.CreateMock<IDbCommand>();
IDbDataParameter dataParameter = mocks.CreateMock<IDbDataParameter>();
IClock clock = mocks.CreateMock<IClock>();
SetupResult.For(clock.Current).Return(new DateTime(2006, 12, 20, 22, 45, 33));
string spToGetItems = "GetLockedRecords";
string spToDeleteItems = "DeleteItems";
getRecordsCommand.CommandText = spToGetItems;
getRecordsCommand.CommandType = CommandType.StoredProcedure;
I could do the same with NHibernate in about three lines, and that would be easy. I'm considerring ripping what I already have and going that route. It would certainly be easier.