LoopNet, the #1 commercial real estate service online, recently launched a new map search service built on Virtual Earth.  The LoopNet solution utilizes Virtual Earth to provide an autorefreshing query for commercial real estate based on the VE Map Control.  That lets their customers use the map as an additional filter in their search, finding only properties in the exact areas they care about.  The unique bird's eye imagery also lets users get a real perspective on 'what it's like there' as they investigate sale or rental properties. 

LoopNet's solution powers over 1000 websites used by over 1.2 million members.  It contains over 2.8 billion square feet of lease listings and $300 billion in sale properties.

A username and password is required for full access to the map search, but they provide details about their implementation in their New Imagery page.

Update 1: LoopNet actually powers over 1000 websites (!), not the 300 I originally quoted.