Like it or not, Vista is coming simply because its easier for IT to manage just one platform. Josh Greenbaum makes sense of this over on the Enterprise Anti-matter blog.
Enterprise IT has never run a popularity contest, and if you doubt that just look at the unbelievably crappy user experience that has dominated enterprise software since the dawn of time. IT runs an increasingly cost-conscious effort aimed at trying hard not to pay too much attention to worrying about how much its users are actually loving their software. IT wants efficiency above all, and will always opt for expediency over technical "correctness", which means that Microsoft's incumbent position on the desktop -- combined with the significant cost-differential between a Mac and a Window PC -- isn't going to be usurped just because Vista sucks.