The X100 will be available in March 2011. For more information, please see www.fujifilmx100usa.com.

Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is a fast lens because it delivers more light intensity (illuminance) to the focal plane, allowing a faster shutter speed. A smaller maximum aperture (larger minimum f-number) is “slow” because it delivers less light intensity and requires a slower shutter speed.

via Lens speed – Wikipedia, the free encyclopedia.

Long Version:

For a detailed guide to the f-stop, see ”A Tedious Explanation of the F-Stop” by Matthew Cole.