Although the iPad has yet to ship, a potentially significant rumor Monday night has Apple already developing a more advanced model. Supposedly corroborated between sources, the new model would be much larger and more likely run software much more like Mac OS X, not iPhone OS. If true, the system would be ready within a year.
The TechCrunch contacts are short on many details, including screen size. However, a particularly large system is less likely due to the nature of the screen. A large screen area would likely need to be strengthened to avoid breaking; it's rumored the iPad was pushed back due to glass issues. A larger screen also brings questions of protection as it may be more prone to cracks or accidental damage.
OS-level multi-touch is equally a concern, as Mac OS X to date only recognizes basic gestures controlled by a trackpad and isn't built for touch. Whether or not Apple introduces this in Mac OS X 10.7 is completely unknown at this stage.