Sign up for an openX hosted account (openx.com) then use the invocation codes for the ad slots to embed into your themes - the majority of users will have no idea what the code is (especially if you omit the comment lines) even if they somehow get into the theme editor.
That way you can also run a whole host of ad targetting stuff - e.g. geo-targeting, language, browser, OS, time of day, day of week, referrer URL, and if you use the relevant data in your user sign up, you can target ads by age, sex and so on. You can also restrict / control the ads by start and end date, number of impressions total, or deliver X impressions between 2 dates with auto-spread recalc daily, and much much more.
The WP ad plugins are great for novices and for single WP installs if you only have one blog, but once you start rolling multiple blogs, and especially multiple WPMU sites, you really need something with the power, flexibility and granularity of openX ... plus you can drop in "back up" ads from agencies, Adwords, eBay, Amazon etc for when you have no direct purchased ads or announcement banners to show - plus you can set display priorities and weightings to only have those show if there's nothing else available.
Plus ....
.... oh shut up already that man, don't oversell it
;) :D
Gaz