This, a thousand times this. IE6 persists in corporations not so much from risk-aversion, but simply from inertia and the lack of a cost-benefit justification to upgrade. Windows XP is the real culprit. XP installers even through the latest service packs never came with anything newer than IE6, so it's going to stick around as long as XP does. And given the frequency that most corporate workstations get reassigned and reinstalled and reimaged, IE6 comes out of the reinstallation casket all the time.
And consider the users as well. These are corporations with thousands of users with no technical expertise, especially corps in fundamentally non-technological fields like say education or food service. These are the users with twelve handwritten steps on Post-its for how to open their timesheet on the intraweb. Even a switch to tabbed browsing can get them lost in their routines. What IT department wants to spend any amount of time upgrading and retraining these users?