Analyst Egle Mikalajunaite of research2guidance has modeled the growth of the Android Market and Apple App Store, and based on his predictions, the former will overtake the latter in August of this year. While we're generally pretty weary of these sort of predictions, the short-term nature certainly makes this much more plausible.
The graph largely speaks for itself, but there's a bit more to be said in terms of specific numbers. First, according to their numbers, the Market has been adding more apps per month since October of 2010. Second, it's since continued to pick up the pace (notice that you can see it ramp up quite dramatically beginning in August of 2010).