If you jailbreak (don't do it without researching it), there's an app called SBSettings that adds a bunch of quick toggles, including brightness, in a snappy drop down menu accessible from inside any app.
Yup, very handy. I have this installed. I still wish there was some custom fine-grained controls that the user can set for their own preferences (based on brightness of their surroundings). I would definitely pay for some app that does this.
Related article here: http://gizmodo.com/5673444/why-existing-iphone-4-and-android...