On Mac OS X, buttons are on the left too - with the close button being the outermost one (whereas in Ubuntu, it's the innermost). Having worked with Macs, it would probably not make much of a difference for me - although some users voiced their concerned about applications that natively displayed buttons on the right despite their configuration, introducing a consistency problem.
However, was moving everything to the left beneficial for the user? Let me remind you Office 2007. I do not need to mention how much heat it had to endure when Microsoft introduced the new UI. Was it really that bad? Maybe, maybe not - I haven't had enough experience with the software to say if the new layout was really defective. For my part, I didn't really like it but that's another story. You may also recall GNOME's X Screensaver dialog replacement a couple of years ago. One thing is certain : in all cases, changing users' habits is a very delicate thing. Users have to see the benefits of it - concrete benefits.
Maybe putting buttons on the left was a good design idea and was truly beneficial. However, if such change is unnecessary in the eyes of the user, such modifications may backfire. And sometimes more than you expect. I mean, it's only window buttons, after all!