Over and over, people trying out the virtualized desktop infrastructure (VDI) approach to enterprise computing (Xen Desktop and such) notice a distinctly poor user experience compared to the traditional rich Windows client desktop.  It seems that taking the desktop away from the endpoint and putting it in the datacenter only punishes the user with an often unusable system.  See this sometimes 'uncomfortable' video from a team working with XenDeskop as they try to log in on a few devices: http://www.brightcove.tv/title.jsp?title=1644064993

And for most businesses keeping the end users' productive is a requirement.  Slow response on the user's PC is too much of an expense for the line of business, not to mention a poor level of customer service, risking your IT department's reputation internally.  

Over time, this may change.  As network scale increases, so can the user's experience, and 10 Gigabit NICs seem to be the next wave of client hardware.  However, the current cost of giving your clients this fast of a network connection is very high.  Check out some of the latest prices for 10GBe that supports Vista:

$641.79 - SMC10GPCIE-XFP http://www.newegg.com/Product/Product.aspx?Item=N82E16833129168

$1099.99 - SMC10GPCIE-10BT  http://www.cdw.com/shop/products/default.aspx?EDC=1387933

This points out a very common refrain you'll hear from me:  "One does not virtualize the desktop OS to save money.  Security is the primary driver."  And yes, bringing the OS into the datacenter avoids having a hard drive outside one ring of your security perimeter, but right now it will cost you plenty to attain that level of security and maintain a high level of user productivity at the same time.