Browser updates from Microsoft have ALWAYS been opt-in and that is the reason you don't see the same adoption rates like with other browser manufacturers.
As a web developer, it's my biggest gripe with Microsoft.
If you have Windows set to automatically update then "critical" updates will be install without prompting the user. However, when it attempts to install IE it will always as for the user's permission. I feel that it's a bit different than a regular hotfix.
