Windows XP and Windows Server 2003 were the first Windows releases to introduce support for Intel's Itanium architecture. Windows Vista didn't come in an Itanium version, and now, the server line of Redmond's operating systems will drop support for it too.
Therefore, Windows Server 2008 R2 will be the last version of Windows that supports Itanium. This means that mainstream support for Windows Server on Itanium will end July 9, 2013, while extended support will end July 10, 2018. In other words, current customers have little to fear.
This won't be that big of a hit for Microsoft, either. Itanium has never gained a whole lot of traction, and since it has been estimated that only five percent of Itanium systems run Windows, it only makes sense to divert the Windows on Itanium resources somewhere else.
Red Hat has already announced that Red Hat Enterprise Linux version 6 will not support Itanium either. All this spells doom for Itanium, which was once predicted to more or less replace x86 outright. Despite massive investments from Intel, as well as a boatload of hype, it was pretty much still-born.
This graph says it all, really.