Google reached a significant milestone for its Android mobile operating system today: 100,000 applications are available in the Android Market.

The company announced the news in a post to its Android Developer Twitter account, nearly two years to the day since the Android Market formally opened for business. An unofficial estimate of Android applications by AndroLib had pegged the number at 50,000 in April, but it's not clear how accurate that was, as AndroLib currently predicts that 150,000 applications are available in the market.

Google is still well behind Apple when it comes to mobile applications, of course, with iPhone, iPod Touch, and iPad owners having access to over 280,000 iOS-based applications at last count. (UPDATED 1:37 p.m PDT - Apple e-mailed in to clarify that on its earnings call last week, CEO Steve Jobs said there are now over 300,000 applications in the App Store.) Still, the milestone is worth noting as Android continues to cement its role as the Apple alternative for developers and consumers.

Before Google made the formal announcement The New York Times posted a story noting the milestone and pointing out some of the challenges that remain for Android developers, such as the subtle-yet-important differences between the hundreds of different Android devices on the market as well as payment issues with Google Checkout.