You can get this functionality in an iPhone app (onOne), but it probably kills your battery, and besides: in photography, more accessories equals more glory. So check out this little doodad — it hooks onto your camera’s hot shoe and beams a signal to a little LCD monitor, where you can adjust aperture, take an exposure, and so on.
The only trouble is that it doesn’t seem widely available, and I’m not really sure what the resolution is. The website claims 960×240, but that can’t be right. Looks like a handy and simple little thing, though at $335 it isn’t cheap. There are surely other versions of this that I’ve just never sought out — maybe it’s time for a little light googling.