Crossing the 400,000 app mark is very impressive considering that it was just this past August when the Market hit the 300,000 plateau. This can be attributed to the fact that there are more than 100,000 developers making apps for Android.
Check out the chart that compares the growth between iOS and the Android Market. While it is clear that iOS was faster out of the gate, both platforms hit the 400,000 threshold at around the same time. The rapid pace in which the Android Market is growing is a testament to the platforms increasing popularity.
At the rate that the Android Market is growing it is not hard to think that they will hit the 500,000 app mark sometime in the next few months. Passing iOS in terms of available apps is certainly not out of the question somewhere down the line.
source: Distimo via Mobile Burn