Are you referring to the halftone screen? If so, it can be done in a few simple steps using Photoshop. As for illustrator, I'm not sure how it would be done, if it's possible.
1. Make a monochromatic image or shape you want to add the effect to.
2. Add a gaussian blur that gives you a good, soft edge, but leaves some definition.
3. Change mode to gray scale, then to bitmap...
4. Choose halftone screen as the method, and reduce the frequency number. Use trial and error to adjust the number as you see fit.