The Photosynth team just posted a web log entry about (part of) why they are moving their database and imagery to Windows Azure (from where it is today, hosted on LimeLight Networks and the like).
They explain that when they launched Photosynth, Microsoft didn't have anything like Azure, but now that they do and it's stable, it's time to use it.
Both the current hosting system and Windows Azure have CDN (content delivery network) services.
(CDNs exist because a longer distance that information travels over the internet means a longer time that it takes to reach you, therefore it is important for websites with a global audience, like Photosynth, to have web servers that are in many physical locations around the world. For example, if they detect that you are in Australia, they can send you the copy of the photos you want from the Australian servers and things should load for your more quickly than if all the web servers were in the U.S.A.)