Hi penant

Are you starting the OS 9.1 update from this page? http://docs.info.apple.com/article.html?artnum=75288

The note says that cookies must be enabled on your browser and that you have to agree to an export license. Sounds like a couple of steps that could cause a problem, so clear your browser and cookie cache (Internet Explorer?) first. The update is big (71 MB), so over a slow connection you can get corruptions, but I just tried it and it verified properly.