A good place to start is the adCenter developer blog: http://adcenterdev.spaces.live.com/
Also, we have a discussion forum specifically for API questions: http://forums.microsoft.com/MSDN/ShowForum.aspx?ForumID=901&SiteID=1
Hope this helps...
Shai Kariv - MSFT
Please note that adCenter API documentation is now online:
http://msdn2.microsoft.com/en-us/library/aa983013.aspx