The growing viability of so-called consumer-grade applications, catalyzed by the growing maturity of the end-user/consumer community, poses the possibility of what you call the ‘unmanaged PC’ model - in the extreme case one could visualize IT enabling a ‘bring your own PC’ scenario, augmented by the use of services such as Windows OneCare for instance.
It is not dissimilar to the mobile phone scenario today - it is not IT that procures and manages the cell phone today - in much the same way it is possible that it will not be IT that procures and manages the desktop of the future.
Yes, there will be always be user populations in the enterprise where the level of configuration, control and lockdown will be mandated by IT - for compliance reasons, for governance requirements, among others.
Yes, obviously you need IT infrastructure for managing identities and relationships ala Active Directory.
But IT could empower the 'edge' by freeing it.
Will it happen? could IT could give every user an allowance - and they get to chose the hardware, software and enabling services.
Obviously, these are my opinions - all speculation here is mine and mine only - not those of my employer.