ISP Planet has cobbled together a news account detailing how many customers some of the VoIP carriers have.
While one has to seperate free downloads and installed clients from paying customers (i.e. Skype vs. Optimum Online) what is apparent is that the cable companies like Charter and Cablevision are gaining significant marketshare in getting their customers to sign up for Telephony Service. How many of these customers are VoIP in a classic sense versus Digital Telephony is unknown, and overtime will likely all be VoIP.
This clearly means that the RBOC's have to worry more about the MSO's than they do companies like AT&T, Vonage, Packet8 and so forth at this time and that the cable friendly philosophy of Vonage and AT&T is a logical move on both their parts.
The list is far from complete and may not be totally accurate as Jeff Pulver pointed out on his blog.
I also wish that the upstarts who arrived on the scene this year would publish numbers. All AT&T has ever said to me on interviews is they are ahead of plan when pressed about CallVantage, while other companies like Broadvoice and VoicePulse have flat out declined to reveal numbers. Also missing are companies like Covad and Call Tower, both of which provide hosted VoIP based PBX replacements. While none of the companies that are keeping their numbers close to the vest have to report them to us, what is clear, as Jeff points out, is that VoIP is clearly installed and the numbers are far greater than anyone imagines.