I have an iPad and I've used it some for photography while traveling, and here's some things I've come up against.

• With the iPad 2, resolution was limited to 1024×768. The new model's 2048×1536 is much, much better, but the screen is still quite small.
• Lovely for finger-painting, hard to get precise control for fine work.
• Can't color calibrate at an OS level. This might be fixed eventually, but that's the current state. You can get gallery apps which use color calibration (there's one from Spyder), but I don't think there's any photo editing / image manipulation tools for the iPad which are color-managed
• Limited software support. There's some pretty impressive image editors for iOS, but mostly they compare to Photoshop Elements, not to more full-featured tools.
• A subset of that but worth calling out: very limited RAW development options. I understand that there are a few apps that do this, but it's not like you can run Lightroom. (Although Aperture is rumored, I wouldn't hold my breath). I haven't tried any of 'em, instead using my camera's built-in after-the-fact raw conversion or simply waiting til later.
• Limited workflow. Apple has weird restrictions on its "photo gallery" API, and apps using the "camera roll" and standard folders can't do things like create folders or delete images. This means they need to maintain their own file hierarchy, which is kind of ungainly. And apps that do that have no good way to communicate their "internal" files with each other.
• Apple's silly $30 "camera connector" kit is required and another thing to lug around (and lose). Android tablets have SD card readers built-in.

Rumor update: Adobe has done back-room demos of Lightroom for iOS, and apparently there was an accidental information leak on their web site, implying that it will be available for a $100/year subscription, which might be worth it. If that happens to include integrated color calibration, that might address most of the points above.