This is an excellent article discussing Microsoft's Live Communications Server 2005 (LCS) which is to be released later this month: Microsoft to Muscle Deeper Into VOIP?
From this article: "Microsoft's end game is to become a telephony provider and give Vonage, Verizon, etc., a run for their money," said one source close to the company, who requested anonymity. "They are going SIP (Session Initiation Protocol) in a big way. And because SIP is multi-media capable (start with voice, switch to video, conferencing, etc. in mid stream), it's a nice protocol for them."
Microsoft is getting back into VoIP in a big way with LCS and will be pushing this product pretty hard. It has some cool features such as built-in instant messaging, VoIP, video, presence, etc. BUT itss major flaw in my opinion is its reliance on Microsoft Active Directory. The complexity of Active Directory is too much for smaller offices, so LCS will not be a viable option. I know the earlier versions relied on AD, but perhaps this one does not, but I doubt it.