A nice round number, just in time for Steve Jobs to trot out at next week’s special event

Click to enlarge. Source: 148Apps.biz

According to 148Apps.biz, Apple’s AAPL U.S. App Store passed the quarter million milestone on Friday, two years and 49 days after it opened.

As of Saturday morning, by this count (which doesn’t include apps available only overseas), there were 251,007 applications from 50,304 publishers available for download for the iPhone, iPad or iPod touch. AppShopper.com, which keeps a similar list, has the total at 253,777 apps, including 24,334 for the iPad.

A 148Apps pie chart that breaks down the list by category shows that books (with 17%) have now overtaken games (14%) and entertainment (11%). According to an analysis posted by Royal Pingdom Friday, roughly 70% of the offerings on Apple’s store are paid apps. On Google’s GOOG Android Market, by contrast, 64% are free.

Below: Royal Pingdom’s paid-vs.-free graph and 148App’s bar chart showing the App Store growth month by month.

Source: Royal Pingdom

Click to enlarge. Source: 148Apps.biz

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[Follow Philip Elmer-DeWitt on Twitter @philiped]