I'm having the same problem (after having big problems with the install in the first place!).
I've found that if I go into the registry and remove all of the references related to SQL Server under HKEY_LOCAL_MACHINE\Software\Microsoft the uninstall would work.
A word of caution though: I had the luxury of working with this problem on a test box that I can scrap if something unforseen happens. I wouldn't want to attempt it on a production system, just in case the registry editing has consequences that I'm not seeing. This was sort of a last ditch effort on my part that happened to work out :^)
I'm still looking at the problem, and if I learn anything new I'll post it.