Forrester's report is titled "Enterprises Warming To Windows Vista", but that's only because they are trying to put a positive spin on the abysmal adoption figures of Windows Vista in the enterprise market. Last July, a report from that same Forrester stated that Vista adoption in enterprises was stuck at 8.8%, compared to Windows XP's 87.1%, and they even called Vista the "new Coke". The new report puts Windows Vista adoption at "just fewer than 10 per cent of all PCs within enterprises."
They are trying to make it all a little more positive for the much-maligned XP successor by also stating that 31% of IT decision makers "have begun their migration" to Vista - but that can mean anything from reading Micorsoft's Vista pages to actually placing orders for new machines, so it doesn't mean a whole lot.
There is absolutely no going around the fact that Windows Vista is massive failure in the enterprise market. To make matters worse for Redmond, with Windows 7 on the horizon, companies have even less reason to migrate to Vista. Why migrate to Vista now, when you can migrate to Windows 7 only a few months from now? This would explain Microsoft's insistence on a 2010 release date of Windows 7, even though every indication points towards a release before the holiday season of 2009.