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If you had not take any pain to download IE 9 till now, dont do it now either. According to the tweet of IE 9 here and here, IE 9 will be rolled out as an important update, Not sure if its going to be compulsory, I guess it should be optional. This push via updated started from 21st March and might take some time to reach all the computers. The first roll out will be to the beat and rc users and according to this tweet it has already rolled out 10% of users.