I have been talking quite a bit lately about SIP, VoIP, XMPP / Jabber and IM interop. I think most of us would agree that if we could get the best of all worlds in one easy to use IM application we would potentially be a little happier every day coming into work.
From my perspective, an avid user of the larger IM services (AOL,MSN,Yahoo!,Google) and a Broadband Telephony services user, I would love an IM client that I could use to get a good quality softphone and a federated messenger app that I can also use with my own IM / XMPP network.
This last part is important for many organizations judging from the conversations I have had with several large organizations. According to them it's crucial to have control over their own IM network. They need to be able to connect to their own XMPP / Jabber and SIP servers for many reasons including Usage Auditing, Black Listing, SPAM control etc.
www.wavago.com does what you are asking for
federated chat and P2P VOIP and it works on dialup phones.