Erin Dalzell sent me a link to a recent Cringely article ("The Best days of Voice-over-IP Telephone Service May Already Have Passed"). Cringely seems to think that VoIP is doomed because it will just be sneakily blocked (or rather, not tagged with QoS.
I think Cringely gives the telcos and big network operators too much credit. This would be non-trivial to implement across entire networks.
Aside from the difficulty or not of tagging all network traffic is the encrypting/tunneling option: much like P2P traffic that will likely start to travel encrypted over common ports (e.g. pretend it's regular HTTP traffic or VPN traffic or...), VoIP has the same option.
If the network doesn't know you're running VoIP, it can't block it.
Suffice it to say, I'm not too worried about the future of VoIP. We haven't seen anything truly interesting yet (well, Skype comes close, but it's closed, proprietary nature makes me not want to like it), and there is lots of room for innovation yet.