Grab your iPhone, bring up Messages, tap on the text field to start a message to someone, then rotate your iPhone sideways. Did a giant scribble box take over your screen? If so, this is for you.
A tiny tip, but one that many people seem unaware of. In the lower right corner of that scribble box is a keyboard icon. Tap it, and your Messages screen returns to its normal, rotated self. And that will stay the default rotation view for Messages.
Want to return to the scribble view? Tap the button again (the label changed to a curlicue).