Admins can rest easy for a bit as it seems there was a bit of a miscommunication by Microsoft's IE team earlier this month. They advised then that IE9 would be pushed out to all users by March 21. As it turns out the software giant is staggering out the rollout of IE9 via Windows Update starting with beta and RC users first. Microsoft's Doug Neal had this to 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.
So in short, it will be a little while before we see in IE9 in WSUS. This will give admins time to prepare, so there's no need panic about not having time to test how line of business applications 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.