Today Apple began accepting pre-orders for its iPad tablet device. Based on analyst estimates, approximately 50,000 were pre-ordered during the first two hours of availability. At that rate, Apple will sell one million by tomorrow. Also, plenty of new details about the device have surfaced.Several different groups attempted to gauge just how many iPads Apple is "selling" today via pre-orders placed through its web site. Victor Castroll, an analyst with Valcent Financial Group, said, "51,000 orders in two hours."

He and others came by their estimates in similar fashion. People placing orders through the Apple Store receive an Order ID number. Earlier today, a report showed that two orders placed about 30 minutes apart reflected Order IDs that were about 10,000 apart. Other checks using the same methodology returned approximate results.

There's at least one problem with this model. Not everyone placing orders through the Apple Store were pre-ordering iPads. Some of them were surely ordering other stuff. That caveat can be offset, however, in knowing that there are people ordering more than one iPad. Two iPads ordered together would result in a single Order ID.

Are these estimates accurate? Hard to say. Based on Twitter chatter during the morning hours today, plenty of people admitted to placing orders for two iPads.

There are other details that have come to light with today's commencement of pre-ordering.

First, the mute button on the top of the iPad is now a screen rotation lock button. Pressing it will prevent the screen from automatically rotating from portrait to landscape and back if the device is moved or jostled around.

Second, iBooks will be able to sync free (non-DRM'd) ePub titles in from iTunes.

Third, the iPad can use VoiceOver screen-reading for reading pages of books out loud.

Fourth, customers who choose to purchased the iPad 3G and sign up for mobile data plans will have on-board account management tools. For example, those who go with the 250MB plan for $20 per month will receive alerts when they've reached 80% and 90% of their data allotment for the month.

Last, the iPad will support Google's Terrain View in Maps, which lets users see the contour of the land to get an idea of the surrounding landscape.