Unfortunately setting the MasterPageFile in web.config is broken for the designer in the current release of VS 2005 (See http://blogs.ipona.com/davids/archive/2005/10/12/3408.aspx for details). So here's a rough and ready simple hack that you can use to easily switch masters for a deployment.
1. Create multiple default.master files organised in a consistent folder structure. I use Masters > SomeMaster > default.master, Masters > SomeOtherMaster > default.master etc.
2. Create a default.master in the root and create all your pages using this
3. Use a web deployment project and create an MSBuild task to copy the master you want over the one in the root. For an example of similar MSBuild task see this earlier post
During development you can easily copy one default.master over the root one too, so you can preview results in the designer and browser easily.
Pretty obvious, pretty clunky, but at least it is a workable solution until the designer gets fixed.
HTH
Ian