A new resource database was introduced in Sql Server 2005, and it is still with us in Sql Server 2008. In both editions, you will not be able to see it via Sql Server Management Studio. This hidden database is required. Without it, Sql Server cannot start.
In Sql Server 2005, mssqlsystemresource.mdf and mssqlsystemresource.ldf are stored under the DATA folder, along with master, model, msdb, and tempdb. In Sql Server 2008, however, the two files have been moved to the Binn folder, where sqlservr.exe executable is located.
By the way, do you know why the folder is called Binn? Why two “n”s? There is a bit of history there. Back in the 16-bit days, all binary files were stored under the Bin folder. However, as things moved to 32-bit and Windows NT came to the market, this new folder, Binn, was created to accommodate that. The extra n was taken from NT, which stands for “New Technology”.