Before I bought my first iPhone, I really didn't care for mobile gaming. Games on small screens and mobile phones were just horrible with poor graphics and junk titles to choose from. The iPhone changed all that for many of us.
If you are like me and have found that on the iPhone mobile gaming is lots of fun, you probably have a gob of apps on your iPhone already. The good news with the iPad is that all of those apps will run on the device. I take that to mean you won't have to repurchase the apps if you have an iPhone and buy the iPad too.
From the photos of the iPad playing Need for Speed Shift, it's clear that the iPad is more capable than the iPhone when it comes to gaming. I am very interested to see if the claims I have heard from early users hold true for poor battery life when gaming. I know the iPhone battery is gone in no time if you play games, so I won't be surprised to find that same issue on the iPad.