Yep, it's Universal Plug and Play (Pray???
). Unless you actually need UPnP, port 1900 can be blocked.
I had to open it up, however, to use some voice functions in Windows Messenger via Internet Connection Sharing through my LAN. ICS runs as a software router, so it needs to be UPnP capable for Messenger to be able to assign ports for the voice functions. At least this is what Microsoft KB said!
