iPad mania has hit since Apple's answer to the tablet became available for pre-order on Friday morning. We already mentioned with glee that the mute switch has been transformed into an orientation lock. Now, sales estimates suggest that virtual iPads are flying off virtual shelves as fast as Apple fans can click "Buy Now," and a number of details are surfacing about iBooks, 3G data services, and more.

Of course, Apple hasn't made any official announcements about sales numbers, but that hasn't stopped clever buyers with a Google spreadsheet from estimating how many iPads Apple is selling. Using order numbers matched up to the time of order, estimates range from about 20,000 to 25,000 iPads sold per hour in the hours after Apple raised the curtain. Assuming the majority of sales are the $499 entry-level model, Apple could be grossing about $10 million per hour. If this rate keeps up, it's possible Apple will sell through its first batch of iPads (set to ship April 3) this weekend.

Supplies are expected to be constrained for some time. Apple is limiting general consumers to just two iPads per person for preorders. However, it is also limiting business and academic purchases to just two per person as well. And initial store stock may be based on the number of preorders that are reserved for pickup at a particular location. If a store doesn't get many reservations, it may not have many extra iPads to sell to other customers. Apple is really pushing preorders for those that want to get an iPad on launch weekend.

Apple is allowing preorders for many of the iPad accessories as well, including the keyboard dock and the VGA video-out adapter. However, conspicuously absent is an option to order the Camera Connection Kit, which allows connecting a digital camera via USB or directly accessing an SD card to download images and videos to the iPad. The iPad's tech specs say that it supports H.264 and MPEG-4 video—no surprise there—but it will also support Motion JPEG in an AVI wrapper, which is still a common video format for many digital point-and-shoot cameras.

It looks as though roughly 20 to 25 percent of iPad buyers are opting for the model with integrated 3G networks for mobile data. That model carries a $130 premium, and will add at least a few weeks to your wait. However, the extra cost and the no-contract data plans are actually quite a good deal. Earlier in the week, we examined some alternate options for data on-the-go using mobile hotspots, and the pricing for hardware and data plans fall clearly in the iPad 3G + WiFi model's favor if you plan on needing to use your iPad while far from WiFi hotspots. Also, if you opt for the $15 per month, 250MB limited option, you'll get warnings when you get close to the limit, as well as an option to move up to the more expensive plan. You can activate and cancel data services at any time right from the device, so you only have to pay for data when you need it.

We also have some good news and some bad news regarding iBooks. The bad news is—and this is something we fully expected—books purchased through the iBookstore will have Fairplay DRM and won't be compatible with other e-readers. The good news is that iBooks uses the standard ePub format, so any DRM-free ePub files can be loaded into iBooks using iTunes syncing. (Apple hasn't said whether publishers will have an option to make their titles DRM-free.) Some even better news is that iBooks can use VoiceOver to read the contents of any book aloud. You might remember that publishers balked when Amazon added a similar feature to the Kindle 2, and Amazon was forced to remove the capability for most books—though publishers can still choose to enable it on a per-title basis.

Finally, one little last detail was revealed on the detail page for the YouTube app: sharing options now include posting links to YouTube video to your Facebook wall.

Our crack Apple hardware review team was up and ready to pre-order iPads the minute the Apple Store came online at 7:30am CT, so you can be sure we will have a full and thorough review as soon as humanly possible after they arrive in Orbiting HQ. Until then, we'll be sure to keep you abreast of any developments.