Hi all
After the question by cajpersonalfinance about customising the administration side of WordPress MU ( http://mu.wordpress.org/forums/topic.php?id=8207 ) , I thought I'd post a little bit of information about the system I am currently working on.
It uses a standard install of WordPress or WordPress MU (currently running on MU 1.5.1 as 3 hours ago) and, using only plugins, heavily customises the adminstration system and menus to remove all the parts I don't want users of the system to see (and stops the user accessing the removed pages by pasting in the url). It then adds the Property management and booking/rental systems functionality.
The idea is to turn the MU system on it's head and offer a hosted administration system rather than a website.
Along the lines of Highrise, Basecamp and Freshbooks, etc.. When a user signs up they can pick a sub-domain but when accessing that sub-domain they are taken to the login page and administration system rather than a blog frontend (though the current version also allows a blog as well if enabled by the system admin).
I've posted some information and an initial screen shot here: http://blog.clearskys.net/2008/05/14/introducing-staypress/