Well that was quick - last I checked the Android Market had just hit 38,000 apps, and now thanks to AndroLib, compiler of Android app statistics (among other things), we can see that the Android Market is fast approaching 50,000 apps (48,321 as of this writing). That's pretty impressive. By the looks of things the Market will either meet or exceed the 50K benchmark in time for Google I/O.
In March, Google announced what we already knew (thanks, yet again, to AndroLib) that the Android Market had reached 30,000 apps. Just recently at Google's quarterly earnings call, the number 38,000 was mentioned in reference to the Android Market, but it's difficult to say when those numbers were compiled. Therefore, the roughly 12,000 apps that make up the difference probably didn't just show up overnight.
There will always be controversy over how many apps are actually wallpapers or other things that people may not want to consider apps. Additionally, there's the question of how many apps are just junk or spam. But this issue isn't unique to Android, nor will it ever be. Wherever you have a market for applications, you'll have developers creating quick and easy (read: junky) apps to make a fast buck.
But no matter how you cut it, Android is surely getting there. Now I'd just like to hear official word from Google. Feel free to leave your thoughts below.