Thanks to The Beer House I was motivated to finally install SQL Express 2008, but not without a little pain. FYI...
You can run both SQLExpress 2005 and 2008 on the same machine - Uninstall your 2005 Mgmt Studio and install the 2008 Mgmt Studio, which allows you to see both 2005 and 2008 databases.
I had some trouble getting this all set up on one of my machines, and found that I had much better luck using the "old style download" from Microsoft instead of the newer so-called "web install". I used this link for the download:
http://blogs.msdn.com/sqlexpress/arc...-guidance.aspx
One issue I had with the "web install" was that 2008 Engine install told me I already had it installed, even though I only had the 2008 tools installed. I'd first uninstalled 2005 SSMS and installed just the 2008 SSMS. This problem went away with the older style install. Maybe it'd have been better to have uninstalled the 2005 SSMS and then installed the whole 2008 shebang, SSMS and engine, using the "web Install". Anyway...
Good luck...