On the Mac if you three-finger tap a word, a dictionary definition appears. I'd like to know if I would be able to get this on an iPad specifically for the Japanese dictionaries.
Yes, on iPad on iOS5.1.1. At the moment, I am having trouble getting the same functionality on iPhone on iOS6.0.1, though. Japanese dictionary is there, but I can't get the English-Japanese one. Your mileage may vary, of course.
EDIT May 27 2013: To restore Japanese<->English dictionary, you will need to jailbreak the device and delete the Japanese<->Japanese dictionary. These dictionaries conflict, and the Japanese<->Japanese dictionary wins almost every time.
I have also asked if it is possible to only temporarily jailbreak an iOS 6.1.2 iPhone 4 for purposes of removing the mentioned dictionary. As of the time of this edit, this has not been answered; I am nonetheless linking to that question in case someone answers it by the time someone reads this.