Microsoft IE9 team made a post to clear the bit of a miscommunication that resulted in painc -- earlier this month they advised that IE9 RTW would be pushed via AU to all users by March 21. Which's actually a rollout via Windows Update for beta and RC users. In short, it'll be a little while before the IE9 comes to WSUS. This'll give admins time to prepare, so there's no need panic about not having time to test how line of business apps are affected by IE9.
In any case IE9 will rather curiously be labeled as an update rollup rather than a feature update, so it should be easier to spot in WSUS.
Here's what Microsoft's Doug Neal say on the matter:
We're working to correct this miscommunication.
To be clear, IE9 RTW released to AU/WU today for IE9 Beta and RC users only. Only AU/WU machines with IE9 Beta or RC will see this update -- and it will be un-selected by default. Which is why there's no broad communications yet (perhaps there should have been).
For machines not managed by WSUS, SMS, or SCCM, IE9 is not yet available since we don't release pre-release content (or, in this case, specifically targeted updates to a subset of machines) to WSUS.
I can't speak to future release dates, but I encourage all admins to deploy the IE9 Blocker Toolkit to unmanaged machines where they need to block the installation of IE9 for AppCompat or testing reasons.