Jensen Harris has a post about how they tested out removing the ability to double click in the left corner of a window to close it, but that people couldn’t get used to it so they’re putting it back. Might I suggest that people didn’t get used to it mostly because Office was the only application that didn’t allow it?

If you want to really test out whether or not people could retrain themselves (and they can, even if they complain about having to) you’d need to remove the functionality from all the windows, not just a few of them. Otherwise, the other windows preserve the user’s habits and it just becomes really frustrating that the interface is inconsistent.

Whether or not you should be able to close windows by double clicking the top left corner isnt’ really important, what matters is that the interface behaves consistently and doesn’t surprise users.