Recently I tried to add a WCF based service channel to my NT-Service to be able to handle client requests from a GUI. I never noticed any problems with that kind of stuff so the more I was surprised as I got that error from the headline... Well the difference was that I usually worked (as developers mostly do ; ) with an useraccount which had administrative privileges. The service instead runs as a Network Service... interesting... even if it would be able to open a port and listen to it - it was not abled to use WCF to listen on an httpBinding. I found a few infos in the web but they were mostly regarding problems under Windows Vista and their solutions were all using netsh - which on my system (WinXP SP3) worked totally different so I skipped those experiments. Finally I found a much better Blogpost which featured some more solutions. On my system I succeeded by using the HttpNamespaceManager and adding the base-address (with that strange http://+:port/ Syntax) with access to the Network Service. So thx for that blog : )
Anyway - it might seem a little strange that a network service program can open any port and listen to it and maybe handle http-protocol by itself but not use some sort of windows-aided http-listening. So this may be more of a feature restriction than a real security issue I think...

Supplemental: I worked around that stuff by now with using a net.tcp binding instead of a Http. So - even it seems possible to add these http-stuff programmatically in the installer - this may be an alternative for someone too.