1) The iPad touch screen doesn't really work well when it is covered with raindrops. The cursor begins jumping around randomly when you touch a part of the screen and applications you want to open may not, while others you don't want to open may pop up. Obviously, this is not the best situation.
A beautiful picture of a bird, taken with an iPad through a plastic bag
2) Putting your iPad in a plastic ziplock bag keeps it dry, and the sound production isn't muffled all that much, but the camera becomes pretty much worthless. Above is a shot of a beautiful Black-capped Chickadee... can't you see it... right there... in the tree? So, for showing students pictures of birds from my Sibley e-guide to birds app, the iPad works fine in a plastic bag, even when it's raining. Getting images to share, however, is a lost cause.
3) When you are trying to keep your iPad dry when it is raining, you are more likely to drop it. Yes, I've dropped my iPad out in the field twice... once when I was trying to get it under my raincoat and once when I was trying to put it away in the shoulder bag I've been using to allow my hands to be free. The second time, of course, I was standing on a concrete walkway, but fortunately, the iPad was housed in my aluminum bodied Logitech case/keyboard, and it was the case, not the iPad that took a bit of cosmetic abuse (see below). So, all is well, though I think a few students did overhear my expletive when it clattered off the pavement.
You can see the dents in my iPad case... nothing a bit of judicious sanding won't remedy.
What's the take home message from all of this. Well, you can use your iPad in the rain, but bring along a plastic bag to keep it dry, and include an underwater camera if you want to take pictures. And then, remember that your hands will be cold and the iPad will be slippery, so keep it in it's protective case and you will be able to use it the next time you need it... hopefully when the sun is shining (speaking of which, I do have a clear protective cover over the screen... it has a matte finish, which helps with looking at the screen when there is lots of sun glare (not much of an issue this semester, I'm afraid).