Here's an interesting -- albeit brief -- HomeToys review of the iPad as a Home Controller in a smart home scenario.
I have not hopped on the iPad train yet, but if I do, I know I'll be looking for ways to wring every last bit of use out of it. A home lighting/security/HVAC control point would certainly be one of them.
As the article indicates, the iPad would best serve the user as a controller when away from the home. With a WiFi connection and the requisite app from the control system manufacturer (you should obviously make sure this exists before purchasing a system), you can perform all those "when-away" operations, like shut off lights, arm a security system or crank the AC to prep the home temp before your return.
Check out the full article for a detailed Pros and Cons list for using the iPad as a home controller.