Most of the large scale operations I deal with have multi-person help-desks.

Use all kinds of scripting logic to push out installs and updates...

Usually have pre-prepared OS images that they load onto new/repaired machines...

Have tech staff at buildings supporting the machines/apps/OS...

It's not as easy for them to simply flip the VISTA switch. Granted when they all agree as a group to fashion a migration plan this whole picture will need to be brought into light. That's hardware costs, support staff training, testing of large app's and all those mini-app's they have. Touch-screens for school cafeteria users, hand-helds for security staff, drivers for barcode, printers, and a whole variety of less common hardware.

When you consider all the new software coming from MS right now - EXCHANGE 2007, new OFFICE products - VISTA - it's truly a major undertaking.

Then there is the time required - most of my customers are school districts - so they have that summer two month period (where everyone else is relaxing) to bite the bullet and attempt the rollout.

Having some machine with one OS and some with another OS in a large environment is cumbersome when all these points are taken into consideration.

This certainly doesn't mean that we won't have a VISTA box in our office for testing our apps in advance. We intentionally keep our UI client code simple so that OS changes don't cripple us - but the effects of a major OS are never fully known until the testing occurs.