+1 - I have a need for this. It may be useful for resizable to mimic the features of slider for steps/stepping.

Well, I needed it, so I tried to hack it. I took a cue from the slider which supports this behaviour. Here are the changes relative to the published 1.5b:

line 35-36:

      autohide: false,
      stepping: 0

Insert at line 388:

		if(o.stepping > 0) {
			var steps = Math.round(mod / o.stepping) * o.stepping;
			val = val + mod - steps;
			mod = steps;
		}

Then pass a new option when building the resizable:

$(".package").resizable({ handles: 'w, e', autohide: true, stepping: 20, containment: 'parent'});

That will make the bars move in 20 pixel increments. There is some strange behavior caused when you move through your step range where the opposite end of the resizable briefly shrinks and then returns to its original size when the selected handle moves another pixel or two. It's caused by the code above interacting with the existing width/height modifier but I was unable to determine how to fix it. I thought some more skilled eyes might see the issue.