No-one knows what will be useful is another great argument for a stupid network.
Personally I believe that 3G and even DSL will be eclipsed by grassroots 802.11 buildout, but linking access points with fibre is still worth doing.
Peter Cochrane (former CTO for BT UK) wrote a fascinating paper (PDF version here) on why telcos won't get it right, that basically comes to the same conclusion.