I often quote that there are over 1000 VoIP Providers, including every Tom, Dick and Harry with an Asterisk box. Invariably, I get asked about consolidation. Well, cable has taken the lion's share of VoIP. Far and away the MSO's have become the giants in the VOIP world.
I think dial-tone replacement companies like Vonage do not have much hope, since cable has been quality control and bundling. Also, cellular is becoming the choice for landline replacement. Oh, and deltathree posted horrible 2Q08 results and may close.
What about the Hosted PBX companies? With the help of GenBand and Broadsoft, Business Hosted VoIP should boom (see Gary Kim's estimate). Hosted PBX is the way to go for profitability and churn reduction for VoIP Providers, if you can control the quality and offer a level of customer care.