I've just downloaded and installed the trial version of MS Office 2007 with the "ribbon" interface. In just a few minutes of playing around with it, I've learned that this is probably the first interface I've come across that has an Adaptive User Interface (AUI).
By adaptive, I mean the interface (in both high and low levels) actually changes based on the context of the activity involved. The actual menu items are not user-controlled - instead they are based on user behavior and their tasks. For instance, additional first-level choices appear when the user is performing, say, a table creation or edit, or a picture or art object creation or edit. Specific functions also change based on the object that is selected for either formatting or editing, resizing etc..
The idea is to have the system use anticipatory design practices to better predict user behavior and flow with the daily interactions. Whether or not the system becomes smarter with use is up to the time factor.
Taken this further, I can definitely see an AUI that has algorithms that actually anticipate user actions before they happen. But how could this be without digging into the human brain? This is where science fiction becomes science fact - and it seems we're getting closer to this every day.
As for actual AUI's being in use, I will have to research this subject more to give me my perspective.
** Okay, so by Adaptive, you can say the interface is adaptive if there's personalization functions (displaying or hiding toolbars) as opposed to what I'm thinking in full anticipatory design model, with use of perceived artificial intelligence.