This is more for future reference for myself, but it maybe useful for others who are working with WIX installers.
I've been building a WIX installer which can handle creating Databases. Now on a couple of occasions I got into the situation where the installer ran correctly but when I went to uninstall it failed, which meant the installer was marooned in the add/remove dialog and I couldn't re-install the msi.
Well after digging around there is a switch on msiexec to force the Windows Installer to put your new MSI in the cache instead of the old one. This then allows you to uninstall (provided of course you've fix the problem with your unistall process). The command line call is "msiexec /fv new.msi"