What do you mean "up the framerates", this is not a computer game, they could display the framerate at whatever they please. The reason it is displayed at 24fps is because its shot at 24fps.
Not really. The reason it is displayed at 24 fps is the hardware cannot display anything above 24 fps for 3D. According to James Cameron, the DLP projection systems in theatres can't handle 48 fps for 3D. He states the current technology can theoretically support 48 fps for 2D, but only 24 fps 3D because it's a stereo image.
To display 48 fps 3D, they'd need to retrofit all the theatres with new technology.
Some people have been calling for 30 fps, but he says that's pointless, because 30 isn't that much better than 24, and would still require new hardware. Plus, 30 is not directly compatible with 24, for backwards compatibility. So, he thinks the next step up should be 48 fps. I agree totally.
However, I'm not sure that's the reason people are getting sick. I think the bigger reason is just the 3D-ness of it, and the very large screen. I don't ever get headaches or sick with 2D movies at 24 fps (aside from Blair Witch).