As you will notice Microsoft Content Ads is like Adsense that display contextual advertising in a webmaster site. Tracking back the post from adcenter blog dated October 18, 2006 here, http://blogs.msdn.com/adcenter/archive/2006/10/18/calling-all-us-content-advertisers.aspx

They are testing it on their own site in msn directories like health.msn.com and others and only available to US markets. There are also who signed up to test the pilot run of these content ads using a form provided by them only if you are in a US Market. I recently checked their adcenter blog on the development of their system in here http://adcenterblog.spaces.live.com and still quoting http://adcenterblog.spaces.live.com/blog/cns!85E824269AB8C30D!317.entry

we are currently rolling out our Microsoft Content Ads pilot—our contextual advertising offering in the U.S. This service allows your text ads to display next to relevant content on sites within the US Microsoft network, such as MSN’s Health & Fitness, Tech & Gadgets, Travel, and Money.

So some US publishers are actually using MSN content ads, and going into their adcenter forum I have come across this post with a form to sign-up to notify if we are eligible or we want an update to this program. The thread here http://forums.microsoft.com/AdCenter/ShowPost.aspx?PostID=1468091&SiteID=44  and of course the sign-up interest form.

So if you think you need those, try it out and let us know the result.