In a recent review of the new iPad on hardware enthusiast web site Anandtech, the site's founder Anand Lal Shimpi revealed that Apple may be working on a physical game controller for the iPad. "I know of an internal Apple project to bring a physical controller to market, but whether or not it will ever see the light of day remains to be seen," noted Shimpi in his review.
Many hardcore gamers have dismissed the possibility of gaming on tablets because of the lack of joystickls, buttons, and tactile feedback, which they believe makes many types of console games (such as first-person shooters) impossible to execute well. In his review, Shimpi says "As smartphones and tablets come close to equalling the performance of current game consoles, I feel like the controller problem must be addressed."
It is worth noting that Android's latest versions (since Honeycomb) include support for both wired and wireless controllers. Several manufacturers make controllers that can be used with Android tablets, though such controllers are typically $50 or $60, which seems expesnive considering the games are usually less than $10 each for tablets.
So far, tablet games have managed to get by without the need for external controllers; designers have been clever at finding ways to put different controls into the touchscreen, and it may just be that gamers need to get used to these alternatives (or they need to get better, or both). Gamers once thought first-person shooters would be impossible on consoles due to the lack of a keyboard/mouse combination, which is still the interface of choice for the best FPS players (as noted in this article on the new iPad).
If Apple were to come out with a good game controller for the iPad, it would certainly get support from game publishers. It would also make it possible to port more games to the iPad without having to redesign the control inputs, assuming that an Apple controller would have all the typical inputs of a console controller. Publishers would still have to port software to a very different hardware platform, but that's something they're used to doing. Will Apple actually do this? About the only thing we can be sure of is that we probably won't know until they ship it, if they ever do.