WinAdmin is an iPhone application which uses the Microsoft Remote Desktop Protocol to allow you to access your Windows workstation or server. The new release, version 1.1.0 allows you to access Windows Vista and Windows Server 2008. The iPhone's multitouch functionality makes this application shine by allowing you to easily scroll around and zoom in/out of the desktop. It is one of the more expensive applications in the app store at $11.99, but for administrators using iPhone it is probably worth it. I ended up purchasing it.
My Windows Vista desktop.
One caveat for Windows Vista and Windows Server 2008 systems; you cannot use the new more secure Network Level Authentication option which helps thwart Denial of Service attacks and malware. If you use a VPN and your internal network is secure you should be ok. The settings in Windows Vista can be found in Control Panel / System and Maintenance / System / Advanced System Settings - Remote tab.
The Remote Tab in Control Panel / System and Maintenance / System / Advanced System Settings.