From the blog post Zillow Integrates Virtual Earth into their website from the Virtual Earth team's blog we learn

Zillow.com was already a very cool site for browsing property values, comps, and other home related information, and now they've added Birds Eye Imagery to their application making the experience even more powerful. This is a great example of how the Virtual Earth platform can be used by third party developers to include unique capabilities into their applications. Once the Virtual Earth map control is integrated into a website, the end user can rotate their view North, South, East, and West to examine a property, unlike other mapping platforms that only provide a single look straight down at the roof of a house.

Congrats to the VE folks for getting such a cool web site using their API.