Clay Shirky has a nice essay on the two main flavors of VoIP and what they mean to consumers, telecoms and new providers.
For some time I've been arguing that VoIP in the Vonage sense won't have a huge impact anytime soon. Some geeks will buy it, but it is too close to conventional telephony to add value and any costs savings are highly dependant on FCC rulings which are coupled with a rather greedy taxation system.
What will dust the wireline telecoms is the other form of service -AOL IM with voice, iiChat AV/Yahoo IM with voice, Skype, Microsoft Messenger with voice ... These aren't conventional telephony, but allow users to do much more without complications introduced by touching the PSTN or the mentality of the PSTN.
The curves on these are difficult to pin down, but I'm going to be really surprised if Vonage type services have 2 million subs (about 2% of the market) by the end of the year. If you look at the demographics of the market segments that might be attracted to this type of service you find some gapping holes.
The IM-ish VoIP (Clay class it Plan B) will take off subversively -- it has already started its climb. The red herring is universal service/coverage. The folks who are in the Plan A camp have this in their DNA. They had a difficult time understanding that cellphone service might be popular - limited coverage rather than cost was the issue. The fact that you couldn't get a signal from 99.999% of the US 99.999% of the time was their argument.
If you have multiple communication paths using different technologies it is possible to assemble something that is good enough. For many people an IMi-sh VoIP and a cellphone are going to be just dandy. The consumer wire line voice business won't drop to zero, but it could easily be down 30 to 40 percent in 5 years - partly due to cellphone migration and partly due to a combination of Plan B VoIP and cellphone serivce (I doubt that Plan A service will be as important)
It will take longer for Plan B like services to take over wireless due to the much higher costs involved. It will eventually happen, but not in the short term.
So an interesting question is who is going to own something like ENUM and will it be a unique service or not (I don't believe that ENUM as it is cast will fly, but something that is ENUM-like may have value).