In his blog post "PsExec, User Account Control and Security Boundaries" Mark Russinovich shares some insight on the capabilities of UAC in Vista...

..."It should be clear then, that neither UAC elevations nor Protected Mode IE define new Windows security boundaries. Microsoft has been communicating this but I want to make sure that the point is clearly heard."...

http://blogs.technet.com/markrussinovich/archive/2007/02/12/638372.aspx