It just happened to me while i'm updating a few apps. I did some research and found an article and follow that way. what you need to do is to un-install the app that you're updating. after that, forced close the Market app and clear the cache. then try again to install it from market. well, it works for me.
by the way, there's also another bug that make Market FC every time when u download an app. if it happen to you, just open Setting>Applications>Manage Application> Market and click "Uninstall Updates".
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