Not to mention that there will be many, many corporate IT departments that won't allow it (or any other update, for that matter) to be installed until they've had a chance to test it against their standard configuraton to insure that it won't break any of their other mission-critical applications.
You'll see that alot in places like financial services and other highly regulated industries. It's not simply a matter of them being overly cautious or dragging their feet -- they could get in trouble with the state or the feds if they allow an update on to their systems they haven't thoroughly
tested beforehand, particularly if it turns out the update causes problems. You won't see too many CTOs willing to put their necks on the line just so the rank and file can play around with the cool features of the latest version of IE.
Said testing does not happen overnight. You could potentially be looking at months (possibly many, many months) before some of these companies get around to letting the upgrade through to their users.
There are still some companies even now that are running W2K or even (saints preserve us) Win98 because they've got some vital proprietary application that requires it and they don't have the expertise (or the money to hire someone) to update it, or their IT department just hasn't gotten around to certifying everything with XP, or they've decided they don't want to incur the cost of updating everybody, or they think what they have works well enough and they can't be bothered to change, or whatever.
At my own company, we had an application that was running on (be sure you're sitting down) Win3.1
, which we only migrated off of less than a year ago
. Granted, we only kept one Win3.1 machine lying around to run this thing while the rest of us were on XP, and we didn't use that box for surfing the web, but still, it was there.
Heck, every now and then I see somebody surfing through my company's site using IE4.
So, yeah, if it gets pushed out as a security update, it will get implemented faster
, but it won't be an "all at once" by any means.