Apple took 50,000 pre-orders for their new iPad tablet computer today. They also revealed several new details. First, the mute/vibrate button borrowed from the iPhone will serve a different purpose on the iPad: it will lock the screen orientation. As one of the main uses of the iPad will be reading, users laying on their sides need not be inconvenienced by the built-in accelerometer switching the orientation from portrait to landscape as they shuffle around.

The iPad will also feature several accessibility options, borrowed from Mac OS X. Users can zoom into any area of the screen up to 5X, reverse the display for greater readability, or have books and other text read to them aloud. Apple also revealed screenshots of a la carte 3G ordering, along with alerts that warn the user when they are approaching the lower plan’s 250MB cap.

Apple’s new product is shaping up to be more more than a “big iPod touch.” It’s eReader app, iBooks, looks like a winner. Plus, with full-sized apps like Keynote, Page, and Numbers (think Powerpoint, Indesign, and Excel) already available, the iPad could truly replace the laptop for many more users than people expect.