Yeah, carriers release updates (with their "tweaks"), after the manufacturer adds their "tweaks".
So, for you to get ICS:
Google releases ICS > Samsung updates the SGSII kernel and TouchWiz UI > AT&T updates the radios and bloatware.
This is the big reason I might take my S2 back. While many users never care to update for the life of their phone, I care to. And I cringed when reading about the update process.