I have two hosting companies and both of them offer the "SiteBuilder" program (which I have never used).
When I install zen on any of my sites, it fails to function because the server intercepts any paths with a directory called "zen" and tries to route them to sitebuilder, which most of the time will result in "page not found". This is simply fixed by renaming the themes/zen directory to something such as themes/zen-blue.
I have no idea how many other people this might affect, but I thought it was worth posting here because if zen makes it into core (and I very much hope it does) then this simple problem might generate a lot of help requests. (It took me a lot of time to figure it out.) It happens on *both* my hosting companies, which both use cpanel as the administration front end (which I understand is very common).
If anyone would like to test this, I have set up a test site at http://s1te.org/test2
The log in/pw are test/test and you can try to change your personal theme to zen.
Actually you don't even need to log in. Just try experimenting with zen in different places in the path, such as http://s1te.org/test2/anything/zen and you will see the problem!