Consider following two images:

Here you can see what I consider as a major UI antipattern – Cancel button has two distinct roles. In the first case it works as a close button on the window title bar – if the user clicks it, the wizard dialog will close and the user input will be lost. In the bottom case the Cancel button is a “Cancel task” button – the task will be stopped but the dialog will be retained. This is pretty confusing for the users as they are not sure if they loose input in later case. I suggest we introduce a new button that will explicitly stop the task but has nothing to do with wizard dialog:

This way Cancel button always mimics Close Window button – i.e. it is grayed out when the wizard dialog cannot be closed. I filed this enhancement suggestion (along with a patch) as bug 287887 in Eclipse.org bugtracker.