As Brian Riggs pointed out in a follow-up post, you really don't find anyone arguing that Microsoft Office Communications Server (OCS) can replace a TDM or IP PBX, feature for feature. That's because everybody knows it can't. This is all about where your investments should be placed as you move forward.
One of our commenters made a point that I think is worthy of further discussion:
Interestingly, the [Gartner] report states that Microsoft OCS is being deployed by customers for remote and nomadic workers, people who spend most or all of their time away from a desk (e.g., working from home, on the road, etc.). For these people you don't need some of the features that OCS is missing today, such as E911, (but is meant to have soon).
That comment may give us a glimpse of one means by which OCS may work its way into enterprise telephony. As long as you have main corporate locations, you'll need PBX features for these. But remote and nomadic work, while certainly something that has been in existence for awhile, could become a lot more important than they are even now. As rising energy costs foster more telework, this segment of workers may become more than the afterthought that they currently are. Enterprise procurements may treat teleworkers as truly part of the extended campus, in terms of feature/function and connectivity requirements.
Does that automatically lead to OCS? Of course not. Your incumbent PBX vendors are building solutions that incorporate presence, unified messaging, video, and the host of supporting technologies that make up UC; and of course IBM Lotus counters OCS with its Sametime.
But the bottom line is that corporate telephony always has evolved, and its continued evolution will take it in the direction of Unified Communications.