There are a lot of photographers that saw the iPad and immediately thought it would make an excellent mobile platform for downloading pictures from a camera, quick editing, and uploading to the Web. Well, there are a lot of reasons why this isn't so. The two main reasons are power and openness: 1) Power: Photo editing requires a lot of computing horsepower. 20 Megapixel, 16 bit images are BIG, and giving you instant feedback when you move those brightness sliders around takes a ton of computing which the poor little iPad/iPod/iPhone just doesn't have. Also, since the hardware is so hobbled with respect to I/O, you have trouble just getting your photos onto the device.
2) Openness: Assuming hardware power wasn't a problem you would still be up against Apple's stance about app development in the iPad ecosystem. As Dennis Walker pointed out in his letter below, "the iPad is meant for the consumption of media, not the production of media content." Apple won't let you read from flash cards and they won't let you access the filesystem. Combined with the threat of having your app pulled if it becomes too full-featured (a full-featured app might be pulled for duplicating Apple functionality), and there's really no way to expect a serious photo application on the platform.
He goes into more detail than I could but his essay mirrors my thoughts exactly. I'm amazed how excited people are getting about tablet computing. I'm a firm believer in the laptop and I really wish Apple would make a netbook. But an iPad isn't the right tool.