In previous versions of iOS whenever you looked at an SMS conversation the keyboard would obscure half the screen, and the only way to get it to disappear was to go to another conversation you hadn't responded to for awhile and then back into the one you wanted to read.
In iOS 5 all you have to do is drag the message down so that when your finger drags into the keyboard, the keyboard will drop out of the way.
[crarko adds: I've confirmed the new behavior, but I don't recall how 4.3 handled this. It certainly does seem a more intelligent method than the other.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110809061451178