With AOL failing at VoIP, and with Yahoo virtually outsourcing to Jajah, with Google not doing much talk over Google Talk, it's no surprise that Microsoft has put itself (and the market) into an interesting "let's not do anything for now" position when it comes to VoIP.
MSFT has had SIP in Windows since the days of Windows Messenger. But the dual Windows vs. MSN strategy back then meant SIP got left out of the mass market adopted platform. While it's part of the OS, there are too many other more significant issues with carriers than a VoIP play for MSFT to worry about in the mass market. That said, their enterprise and small business plays seen to be gaining steam.
My view is the world doesn't need Microsoft to be in the VoIP business. They just need to enable it via their OS and their apps.