Instant Message clients are becoming incredibly important and will be the eventual winners in the VoIP sweepstakes. This epiphany came standing at the Microsoft booth. Om Malik's Broadband Blog: The Incredible importance of Instant Message
Sorry, Om, I completely disagree. There is little or no money to be made off soft clients, IM or otherwise. Voice is free today on these platforms (have you used iChat? -- it's crystal clear, and "just works"), why do you think anyone will be able to charge for it in the future?
People will still want actual "phones", even if they are IP phones, that are separate from their computer. Think of how often peoples' computers are unusable. Imagine the calamity if this knocked out their voice communications as well.
Also, open standards will eventually win the day. Jabber's XMPP is a likely winner, and because of strong telecom industry support, SIMPLE will likely succeed as well.
I'm sure that there will be lots of use of softphone voice, but it won't come at the expense of other types of VoIP, but will be a compliment.
Another example scenario: imagine that the gang at FWD hook up with a Jabber gateway and a little software magic. Potentially, they could offer access to (at least) all Packet8, Vonage, and FWD customers, incoming and outgoing. If someone were interested in running PSTN gateways in a few places, this could extend to any pc-to-phone calling.
Thanks, I'll stick with my IP phone for the time being. Feel free to iChat me, though.