I've cleaned up Weber grates by putting them in a hot camp fire. I'm nut sure if you want to build a fire under them right in the cooker. The problem would be getting them hot enough from edge to edge since more stuff tends to accumulate near the outer walls. The stuff that accumulates on the grates is mostly grease and burns off pretty easily. I've heard some clean them with a weed burner.
As with other cleaning methods you'll want to oil the grates soon after brushing the ash off as they will rust easily (unless they're stainless steel.)