Many a start-up I know liked to use Vonage as their pipeline to their developers in far away places. But with the arrival of Skype which combines IM with Voice, they have made the switch. Here's one more example.
When I think about it, Vonage had the XTEN softphone about as early as anyone including Jeff Pulver's Free World DialUp. Instead of using the XTEN software as the core and wrapping an ICQ like IM tool around it, Vonage went on to be a broadband replacement for the PSTN telephone and everyone else has followed suit.
IM is a key behavior changing application--you can ask if someone is there, can they take a call. Heck, you can even leave a message that way. Skype took this behavior and added voice. Now they are adding users at the expense of regular and next-gen phone companies.
Perhaps one of the real players in the game will take Skype's lead and integrate IM into their VoIP platform. Yahoo also already has.