Google doesn't disclose Android Market figures, but AndroLib and others track what apps are added and at what rate. Looking at the different statistics, it's clear Android development is on the rise.
Given the success of the Droid, November was unsurprisingly the biggest month for both new apps in the English language Android Market, and also the biggest gain month over month.
Other interesting tidbits:
Games account for 15 percent of all applications in the Android Market. That's pretty similar to the App Store's own statistics, although the methodologies for categorizing application type might not necessarily be the same.
When it comes to app distribution, 30 percent of apps have been downloaded fewer than 50 times. Only a very small percentage of apps have been downloaded more than 250,000 times, which might indicate that creating a "hit" application for the Android Market requires more nuance than for the iPhone.
2010 is going to be a big year for Android, what with more handsets popping up -- including the Nexus One -- and a continued industry shift to mobile. Twenty thousand apps is a nice way to kick things off.