Sometime this week, the number of apps written or adapted for Apple’s (AAPL) iPad hit the 100,000 mark.

It’s hard to say exactly when. On Thursday afternoon, MacStories‘ Frederico Viticci noticed that the number posted on the iPad’s App Store (under Featured/Release Date) had rolled over to 100,161.

That’s curious because when we checked on Sunday with 148 Apps‘ Jeff Scott, who keeps his own count, he already had a total of 102,994. Meanwhile, AppShopper, which uses a different counting algorithm, is reporting a number that’s 6,005 higher than Apple’s.

No matter. It’s a lot of apps. At least 35,000 more than the 65,000 Steve Jobs bragged about at the end of March.

And how are the iPad’s competitors doing? That’s even harder to say. A search of Google’s (GOOG) Android Market Thursday turned up about 1,200 free Android tablet apps and 145 paid, or roughly 1/75th as many as the iPad.

Thanks to TNW for doing the iPhone App Store comparison.