just fyi, it didn't work for me.
I tried to install SQL Express 2008, and used "servername/sqlinstancename" as the server, then the user and pass that I setup and it said that it couldn't find it. I'm going to try SQL express 2005 soon, but I fear that I may run into problems because 2005 is already setup for WSUS and Sharepoint on SBS 2008 boxes.
Oh well, I'll give a few more things a whirl and post back when I find out something.