IE 7 is coming and it will be delivered as a required update by Windows Update.
This is another data point when considering whether or not to build LOB apps that are only wired to work properly in a specific browser. The day will come - it may take 5 years but it will come - when the browser you certified your business app to run in will be upgraded to a new version - and it won't be on your schedule, it will be on the browser publisher's schedule.
Going forward the IE team is promising to release new versions at a much quicker pace - which makes this issue a more real concern than it has been for the past 5 years.
So don't do that... if it needs to run in the browser, then make it a standards based app that is coded and tested to run in all the modern browsers and this won't be as critical an issue. Your app won't stop working until you get out a version that works in the new version.
If it only runs in IE 6 - then make it a smart client instead. You won't have to manage your release schedule around that of the browser publisher.