I've been having troubles with Notification Centre locking up my whole computer when trying to open it until I force quit the Dock process (which would take anywhere from 5-10 minutes), upon which it would then relaunch and work correctly for maybe 15-30 minutes before the problem resurfaced. I've tried almost every solution, including resetting the ACL & home directory permissions, but nothing has worked. Until now. For some other reasons, I decided to download the OS X 10.8.4 combo updater and install it over my current install of 10.8.4 (which I've done delta updates for since installing ML). Not only were my other problems fixed, but I've also noticed that NC works exactly as advertised all the time now. Going on 3 days and not a single hang or crash. I'm not guaranteeing that this will fix things for you, but it certainly did for me and I hadn't noticed anybody suggesting it before. So give it a shot if you're having the problem and see if it works or if it was just a fluke for me. You can get the OS X 10.8.4 combo updater here.