We haven't been covering the whole "Marketplace hits XXX" for awhile now, mostly because it got old, but we're returning for 'the big fiddy' aka 50,000. According to All About Windows Phone, the app store is now at 50,126 items with 17k apps being added within the last 90 days. In fact, though it took a year to hit 40,000 apps in the Marketplace, another 10,000 were added just in the last 40 days, which hints at a strong improvement in rate-of-growth.
However, some caveats do apply here. While there are over 50k apps, not all are available in all regions and some have been pulled by the publisher or Microsoft, meaning they are no longer available. In that sense, the numbers are a little more sobering: "US (42,655), UK (40,305), France (39,235), Spain (37,027), Italy (36,944), Germany (36,958), Australia (37,024), India (36,752) and Singapore (36,922)".
All About Windows Phone also points out though that average number of submissions is now 265 a day which is almost double what it was a month ago when it was just 165--that increase in submissions is most likely due to increased awareness since the Gen 2 devices launched, especially Nokia's push in Europe and Asia.
Other interesting stats note that 58% of apps are free on the Windows Phone Marketplace, with 14% being paid with free trial and 29% are just paid. This figure puts WP7 right in between Android (69% free/31% paid) and iOS (43% free/57% paid).
Finally, rate of growth is important to look at with Windows Phone taking 14 months to reach 50k while Android took 19 months and iOS did it in just 12. The Android numbers show how even a slow-start can lead to explosive growth when all the right pieces are in place (e.g. device availability, advertising, etc.) while iOS was a bit different since it already had around 4 million users even before it launched the Marketplace (that's actually quite impressive). Sill, the Windows Phone Marketplace numbers show very strong growth and it is still very much a competitor against Android and iOS--given enough time.
Source: All About Windows Phone; Thanks, James, for the tip!