We've always filled our patios with crusher fines ie "breeze" and sometimes, as Paul mentioned, with 1 part Portland.
The crusher fines set up semi hard, but when mixed with the portland, the joints become almost mortared. You just need to be careful to sweep all the portland off the flagstone before wetting. I've also noticed that the portland mix tends to dullen the color of the stone a bit before mother nature cleans it up.
Here is a patio we did this spring:
Flagstone Patio
Nick