Although I found this at TreoCentral, it's designed for any device, WM5 or higher. It apparantly allows filtering by type of call (i.e. known number, unknown number, blocked number), specific caller, specific catagories, or by profile, which can be custom designed. Rejected calls go directly to voicemail and still show up in your call log.
http://www.treocentral.com/content/Stories/2276-1.htm