Business Week Online (registration required) has a very detailed account on the possibility of Microsoft becoming a telephony powerhouse.
I've already played with a hacked version of Windows Messenger 4.7 and Webley Systems voice over IP capabilities for over a year, and while the product is not yet in commercial release, the capabilities of always on, always connected was never more obvious. Sure the NAT issues surfaced with certain routers, but over a year ago the ability to dial, route calls to cell phone, traditional PSTN or IP SIP address was working flawlessly.
But nothing was more exciting than sitting at a Starbucks, laptop running, when a call came in and a conversation ensued, over the laptop. I felt the same way Alexander Graham Bell and Watson must have felt, when they had their first call.
Now, sitting at a hotspot, making a phone call is almost standard. I've made calls over wireless such as GPRS and EVDO. The big drawback, is bandwidth on the upload path.
But all that said, working with an IM (instant messaging client) like any of the messengers (AIM, Yahoo or MSN) for voice and video, or any of the tools like them, are really basis of the next phone device. As laptops become smaller, with more faster and powerful processors, the idea of only a phone will become a thing of the past!