Matias makes a folding keyboard for the iMac that can also be used for the iPad. The only problem with it is that the enter key on the number pad doesn't work with the iPad. They said that they would have a new model out this past January, but I never went back to check.
I have used the bluetooth keyboard that came with my iMac with my iPad. It can not be paired with both computers, but the radio range of the keyboard is limited and I generally don't use my iPad when I am in the same room with my iMac so it was never a problem.
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