According to data Pocketnow referenced, it took Apple 4.7 months to reach this level and Android 11 months. BlackBerry App World launch in April 2009 and has about 20,000 apps. I am not a huge fan of just counting up apps as a way to decide who wins, but with such huge selections the chances that the apps you like are available is much greater.
There are junky apps on every platform and if you want you can throw out all kinds of themes, ebooks, etc. that shouldn't really count as apps. However, it is important to check the trends and if Windows Phone Marketplace continues at this pace then things will be good for users. The quality of Xbox LIVE games on Windows Phone 7 is fantastic, as are hundreds of others that take advantage of the slick Metro UI. For example, take a look at Kevin Tofel's thoughts on Runkeeper for WP7 to see why he likes it better than his iPhone version.