In reply to Richardhenton:
Seems to be some confusion on here.
You don't need a permit to stay overnight on a climb - full stop.
You don't need a permit to do any climb in The Valley.
You do need a permit to go up the tourist path and cables to the summit of Half Dome (think 300 million Americans wanting to do that trail - needs some kind of regulation)
You don't need a permit to come down from the summit of Half Dome.
You do need a back country permit to overnight anywhere else - which you wont get for Yosemite Valley - they don't want people dossing in every bush, and they do enforce this, at night with thermal imaging kit etc but there is only so much ground they can cover.
However people do bivvy on the top of the big walls after topping out (its safer to let people do that than have them come down in the dark), and people do bivvy at the base of routes but sometimes get caught and told off.
That about covers it.