Motorola Mobility, on its blog, has done a great piece explaining the steps it has to take before releasing any upgrades to Android 4.0 Ice Cream Sandwich. That includes getting the source code, merging it with its existing frameworks, bug-testing, getting carrier certification, doing an initial pre-release with some of us in the real world (those are those "soak tests" we hear about) and, finally, releasing the upgrade. No small feat, to be sure.
We still don't have a hard time table for when we'll start seeing Motorola's devices get their updates, though the manufacturer does remind us that the Droid RAZR, Xoom and Xoom Family Edition, and Droid Bionic will all receive updates to ICS. And that's just for starters, most likely.
Source: Motorola; Thanks, Durthquake!