Part of the trouble stems from the fact that ballistic coefficients aren't currently used in the shooting community quite as they were originally defined. It serves a purpose as it is used, but it has deviated from the fluid dynamics based definition.
As it is derived from sectional density and form factor, and sectional density can in theory be infinately high (taken to the extream, and you have a wire), there is no 'highest possible' b.c. built into b.c.'s definition. However, rifle bullets with b.c.'s much above one are on the edge of practicality.
If you want something more technical than Chuck's writeup, check out: http://www.exteriorballistics.com/eb...ed/5th/221.cfm