Well, considering that like many businesses that rely on specialized pieces of software to function (mine in particular being a law firm), we have held off on deploying both XP SP3 and not even put thought into Vista because our document management software and change-tracking/metadata scrubbing software are incompatible with anything above XP SP2 for the moment.
We can't keep entirely up to date because it breaks the software my firm relies on, and replacing them isn't an option. From my experience at the law firms I've worked at, they move at one of two speeds: slowly or not at all.
MESSAGE ACKNOWLEDGED -- The Pershing II missiles have been launched.