Not exactly anything to make Apple or Google shake in their boots, with each of their App Stores and Android Markets totally over half a million apps each for iOS and Android. That said though, Windows Phone Apps have grown substantially from where they were just 6 months ago. If anyone remembers how fast the Android Market grew, it was a slow start for them as well. Apple soon learned that there was something to be feared from a platform like Android, as developers continuously added apps to the market, and Android smartphones continued to sell.
With Microsoft's Windows Phone 7, there hasn't been huge adoption for their phones just yet, but that is also expected to change within the next 6 months after Windows Phone 7.5 Mango update is mainstream, and they begin working on the next update to the OS in 2012.
Currently shown above, The number of apps added to the marketplace over the last year is showing a big growth curve. At the current pace, the marketplace could have over 100,000 apps available in the next 4 to 5 months, and at 250,000 apps by mid 2013. Over half of the apps in the WP7 Market are free as well, giving many people a good reason to start thinking Windows Phone again.