The very insightful Stuart Dredge over at Mobile Entertainment has compiled an amazing primer on all of the numbers behind the app market (which you can also see by clicking the "Read More" link, although it's not viewable on an iPad or iPhone). If you want to know anything about how many apps are rolling out of the store, how many apps are installed on the average device, or what kinds of apps those are, Dredge's presentation will tell you. It's a really excellent compilation of everything we know about the App Store and the app market at large thus far.
I don't think anything here is new or really that surprising. We already know that Apple is, by far, leading the race on number of apps (both created and sold), and we've heard before that games tend to do very well on the marketplace in a number of demographics. The end of the report has some great numbers, too, on standout applications like Angry Birds and the eBay app.
Otherwise, it's basically a big round-up of a lot of the stats that we've heard so far. But having them all in one place gives a nice overview of just how big this little software ecosystem has become.