I wanted to bump this because it's been an annoyance I've had with Android FOREVER! I love the Android platform and the more open system, but if I recommend an Android phone I have to also say that the picture taking is crap.
To my knowledge, Vignette is the best camera app for Android. It's a great app and has lots of nice effects, but it doesn't have exposure lock or selective exposure. To change exposure on Vignette: use gimmicky hold and drag to options, scroll down to exposure, then adjust +/- to where you want the exposure to be. If you didn't guess correctly, you have to repeat this awful process.
"Use a DSLR" seems to be the appropriate response...or maybe "get an iPhone." ;-) It's almost annoying how many great iPhone apps there are that are better than ANYTHING on Android: the stock camera app has better exposure control, Camera+, Awesome, and Instagram (not sure about this one...just a guess).
That's my little rant.
I keep hoping that an update to Vignette will add better features like this, but they're always just bug fixes for new phones. Maybe if Instagram ever comes out, they'll actually try.