Could be that it's not a memory leak. It's the feeling I had since with me it only seemed to happen after some time of usage.
That having said. Does anyone of you also have the Adobe Flash plugin installed? I had until yesterday morning, and ever since that moment I haven't experienced any problems with Opera anymore. And I have used it frequently since. I noticed that apart from the Opera update I also recently installed a Flash update so I wonder if that could possibly be the cause for my Opera troubles.