Steve Jobs has just announced the iPad, calling it a "magical device at a breakthrough price". Apple says the iPad runs all applications currently available on the iPhone App Store, plus apps customized for the device.
Here are the official specs direct from Apple:
Price: $499 for 16GB, $599 for 32GB, $699 for 64GB. If you want 3G that's $130 extra up front, plus $14.99 for 250MB/moth, or $29.99/month for unlimited data.
Shipping is expected in March worldwide for the WiFi model, April for the 3G model. Unfortunately you can't pre-order one right now pending FCC approval.
The iPhone 3.3 beta SDK with support for the iPad is available now from the Apple Developer's web site. It includes an iPad simulator so you can begin testing right away. Similar to Android, you write one iPhone/iPod Touch/iPad app that runs on any device but adapts its user interface to the user's resolution.
If you try to run an unmodified iPhone app on the big screen, it will either display in a small window in the middle of the screen or the user can zoom it up to 2x size. Zoomed apps won't appear as crisp as apps adapted for the iPad, of course. That's because original iPhone apps were designed for a resolution of 480x320 pixels. If you blow that up to 2x the size, you have 960x640 but every dot is 4 times bigger (and blockier) than it was originally.
See also: Live coverage of the announcement.