Stay away from sub clips in Final Cut Pro HD they are horrible horrible horrible and will drive you mad. Especially if you came from Avid. FCP treats them as MASTER CLIPS. It all goes back to the terrible media management that FCP has.
My recommendation is to use markers to make your “subclips”, at least that way you can always find where the subby came from so adjusting things is a snap. Remember you can load markers as if they were subclips right into the viewer or even the timeline.
Let’s hope subclips and all other things to do with Media management are fixed in version 5.