Maybe everybody already knows this, but I missed it in all the new features of iOS 5. The iPad mail app adds a new gesture to show and hide the message list window when in portrait orientation.
I always use my iPad's Mail app in landscape mode because I get to see my message list in addition to the message detail. And for some reason, I find it tedious to push the mailbox button at the top left of the screen to see the message list. iOS 5 added the ability to swipe right anywhere on the message window to pop up the message list and to swipe left to hide it again. Nice! Not having to move my finger six inches has somehow made portrait mode much more bearable for me.
[crarko adds: Well, I didn't know it either, so here's the hint. I also was a landscape mode only iPad mail user.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120120060035517