Keith Robinson shares an interesting experience of creating a Movable Type intranet for an hospital, dubbing the exercise as pushing Movable Type beyond the blog (Matt Haughey, along with others, has done that one before). Digging into flexibility and scalability issues, he rightly notes that, even so MT creates static pages by default, the fact that it generates code from templates when posts are created or modified, not when they're viewed, allows one to use whatever "dynamic" technology afterward. MT will happily generate code that include PHP, ASP, JSP or whatever server-side language is used to perform the dynamic rendering of pages.
On the issue of tweaking the admin interface, I remember having seen something similar done for the redesign of Adaptive Path but can't find where anymore (unless it was on a different project but still, this is another great example of experience sharing on the same subject, plus you can see the resulting site!)
I should mention that MT is certainly not the only weblogging tool that one can "push beyond the blog" and use as a CMS. pMachine, WordPress and surely many more are interesting candidates. And we're still waiting for the now delayed MT Pro...