Here's a cool little update: Microsoft has pushed out, seemingly in addition to the AppHub features, changes in categories to the Marketplace for Windows Phone.
Simply launch your Marketplace and head to Apps. You'll see a few changes including replacing "and" with "+" signs, making things surprisingly easier to read. In addition, we now have a dedicated "Books" and "Business" category (we're not too sure if the latter is new).
We bet this has something to do with the AppHub's "private beta market" launching tonight as well as MS just wanting to further divvy up their 26K apps they have available. Let us know if you find any other little changes that we missed in comments!
Edit: Games too, have new, more detailed categoreis as well, including things like "shooter" "classics", "strategy + simulation", etc. Very nice.
(Thanks, Tahiti Bob, for the heads up!)