See if adding http://boren.nu/downloads/preview_theme.phps restores the preview_theme as intended.
Other than that I'm going to rain on your parade.
In my install I use userthemes and theme-editor to make a copy of an entire theme to mimic another but only to add/edit page templates, or change the header menu, remove the comment form and tinker with post meta links, sidebars, etc., all to use WPMU as more of a CMS for the courses I teach and resources/texts/blogs I publish. No one else has access to theme-editor and I rarely edit the css at all, ironically. Blogs have access to userthemes, blogs of which I am the only Admin.
A couple of years ago I had rigged the theme-editor to be css only and allowed many more users a taste of CSS editing,(I had no control of ftp access to blogs.dir while in classes back then either) but it caused me more work than I needed, and produced hideous(aka "clown pants") results generally. The more I invested in discovering nasty code to scrub, the more paranoid I became in allowing users access to CSS editing at all. So I've stopped letting users tinker and instead upload hundreds and hundreds of well tested themes instead.(Thanks to FARMS for the first 100).
As you see I haven't kept up with the changes to WPMU to keep userthemes error free.
Indeed the ("new") preview functions built into Wordpress(since 2.5?) render the previews as I've coded them useless. The plugin needs work, but it is not on my todo list until after a smooth change to WPMU 2.7 or later.
To be and absolute pessimist, I'd rather invest my time transforming my favorite themes into themes with special functions pages like Mandingo, Tarski, K2, which allow an array of style/colour/layouts to users in a way that is 100% worry free to me as SiteAdmin.