I have a couple questions regarding the sensor. Is there a real noticable difference in image quality between an APS-C and 4/3rds sensor?
They are close. In practice, the best 4/3 sensors currently produce output that looks a lot like APS-C sensors of the previous generation.
Here is a comparison of the output from the Olympus E-PL1 and the Nikon D-3000.
Are the Fuji sensors a different breed than sensors from Sony and Panasonic, as far as their real world performance goes?
As mentioned earlier in this thread, we
do not know what sensor the X-100 is using, and it's the first APS-C camera released by Fuji in five years. For this reason all bets are off – we just have to wait for release of the camera.
That said, most contemprorary APS-C sensors are more than adequate for general photography under a pretty wide variety of conditions, i.e., those that one might expect to encounter whilst using a camera like the X-100.
I think it's reasonable to expect a sensor on par with the one in the Nikon D-90 or the Pentax K-x.