forgive me for my terrible english but i wanted to share something with the community and i hope it can help
--------------------------------------------
well , i'm no coder so i can't participate in the wonderful volunteering work done here but i have something to share that might help some of you monetizing your WPMU installation i'm planning to do on my own WPMU installation
it won't bring you thousands of US$ but it will pay for your morning coffee at least
no user will like to see advertisement on a blog s/he gets from you while s/he can get it without any ADs with much more stability on wordpress.com for example.
so i think it will be of some benefits if you put your AD in the user's dashboard - specially if you r running niche blog host - which means more targeted advertisements to be placed
just open wp-admin folder then admin-footer.php and admin-header.php to put your advertisement code there
i didn't try google adsense but it will not pay much on those position cuz of the crawling of contents will be away from your niche , so it will be good places to sell your ad placements directly - maybe to use AdBrite with reserved price or affiliate marketing there -
it won't pay you that much unless you have thousands of users so you can sell your CPM
-------------------------------------
i want to suggest somethings which in my opinion is must to have in WPMU installation to improve it
1- in Core : to get an option which makes the site admin control the new signups - approve , disapprove , block , spam - from within site admin panel the same as disabling new registeration
2- mu-plugin : recaptcha , http://recaptcha.net , it's used by big companies , facebook,twitter..etc , and it's on no huge load on ur server as it depends on their API - hope i got it right - for the current time i'm testing its single WP plugin as mu-plugin on offline site and it works for comments but don't know how it will act when i go online and i think it will be wonderful if it could be put on registration forms, for those who don't know how it looks like and how it works you can see it here http://recaptcha.net/learnmore.html and it's working to improve the http://www.archive.org/ project which is something good to be with , and they have a mailhide spam protection too http://mailhide.recaptcha.net/ so i think it's a big deal for WPMU community to have it working on COMMENTS FORMS all over the system AND signup form , forgot password, and after X number of wrong passwords provided by the user
you can get more information about their plugins here http://recaptcha.net/resources.html
i know i write in a terrible english but i hope you got my idea