As tablets like the Motorola Xoom begin to nibble at iPad's heels, attention has once again turned to Apple. With the company widely believed to be announcing the iPad's successor at an event in San Francisco tomorrow, many Apple fans are expecting a wide array of groundbreaking changes to the iPad's design. But that's not likely what they will get. Instead, Apple is likely to release an iPad that is more of an evolution than a revolution, a small upgrade to an already-winning formula. Here are the most probable upgrades.