Unlike with ArcGIS Server services, there isn't a server that you can connect to in ArcCatalog to access the Bing services. The Bing services are accessed in ArcGIS Desktop using a set of read-only layer files that ESRI generates with the server URL information built into it, and this server information isn't exposed in the Source tab of these layers because you can't connect to it via ArcCatalog. You can find these layer files, along with a map document and a globe document that contain all three layer files, at this location: http://links.esri.com/en/Bing. These work in 9.3.1 and also in version 10 pre-release and the upcoming version 10 release.
I've been using the read-only layer files that you mentioned for a while now. I just find it so much easier to access the other ArcGIS online services through ArcCatalog and was hoping the Bing services would be available that way as well.
Just a small suggestion for if you guys decide to add that functionality in the future.