I found a ton of help on Google for this “Service Unavailable” issue on the OMA virtual directory for Exchange 2003. Unfortunately, this was not something readily available. Several sites will explain the proper config for all the virtual folders in IIS, which you should obviously follow, but they don’t mention one little thing… OMA uses ASP.net 1.1. If you go into the OMA properties in IIS, change it from ASP.net 2 to ASP.net 1.1. After that, it all worked beautifully!
Oh, and by the way, I ran into this issue while setting up some Windows Mobile devices with ActiveSync and Direct Push at a couple clients, one was running IIS5 on Windows 2000, and the most recent was running IIS6 on the Windows 2003. The solution was the same on both of them.