We're constantly bombarded by mobile industry propaganda that people are "cutting the cord" and moving to become mobile-only..... particularly in the US, there seems to be a perception that, to quote fellow blogger Carlo Longino "the landline's days are numbered".
Now absolutely there is ongoing fixed-mobile substitution in terms of originating call minutes, although the difficulty of tracking VoIP minutes (especially within VoIP communities like Skype or internal corporate PBX networks) muddies the water. Certainly, most of my own calls are mobile-based.
And yet.... perhaps reports of landlines' demise have been exaggerated.....
I just noticed, for example, that France Telecom's recent results has a KPI spreadsheet showing its base of landlines has been rising, slowly but consistently, for the last 5 quarters, up to 34.2m at end-07 from a low of 33.7m in mid-06. The growth has largely been driven by wholesale connections - ie particularly used by its competitors for triple play (with VoIP) and naked DSL.
I think it's definitely fair to say that the PSTN is slowly dwindling. And I don't see an awful lot more copper being installed. But taken together, fixed VoIP / triple-play connections, fixed-DSL, and increasingly fixed-fibre still has legs, in my view.