Financial issues could also work against Vista adoption during 2008. The sub-prime mortgage meltdown may foreshadow a recession; at minimum, the financial services industry is certain to scale back its computing needs. Even companies that budgeted a Vista upgrade in 2008 may reconsider the decision if the economy turns sour.
There's no doubt that XP is the current king of Windows, and it will stay that way for a while. Microsoft's XP support continues through the end of 2011. Microsoft is set to deliver XP Service Pack 3 in the first half of 2008. It's not set to be as revolutionary as Service Pack 2, so it shouldn't be as tough to deploy, but SP3 includes more than just a bunch of security patches that are already available. After many years of tweaking, tuning, and training, XP is finally living up to the slogan Microsoft coined: "It just works."
Given the slow adoption of Vista, an uncertain economy, and customer contentment with XP, it's becoming a safe bet that Microsoft's XP support will not end in 2011. Too many critical customers will be running XP in 2011 for Microsoft to even consider dropping what they call Extended Support, which includes security patches. There's even a precedent: Windows 98. Support for that OS was originally supposed to end in January 2004, but Microsoft announced that they would continue support until June 2006.
If history replays itself, Microsoft will wait until the absolute last minute to announce that XP support will be extended. In the case of Windows 98, the announcement was made just weeks before support was to expire. If customers know that XP is safe for years to come, they'll have even fewer reasons to upgrade to Vista, and that list is already too short for Microsoft's comfort.