We're switching a major project over to SQL Express 2005 from MSDE 2000, and in the process of doing so, I came to several discoveries.

• MS Isn't releasing any MSI Merge Modules for 2005. Why? No idea. Appearently, that great MSI technology that they insist everyone else use isn't good enough for their own products.
• With InstallShield anyway, there is no prerequisite support for Express 2005 in IS 11.5 and earlier. You must move to InstallShield 12. Joy.

I did, however find this article about sqlexpr32 command line switches which should allow you to perform the Express install from within a normal MSI install by just running the exe with the proper switches. Not ideal, but it's something.