I was writing a long email to a friend, and I wanted to copy/paste a URL into the message. The URL was already in another open tab, so I simply switched over to it, waited for it to reload, copied the URL from the address bar, then switched back to my Gmail screen.
...and my gmail new message window was blank. Poof! My unsent email was gone.
Turns out I had a 400 comment monster thread loaded in another tab and the iPad seems to not do so well with memory allocation for rendering all your open web windows, clearing out and reloading them whenever you re-request them.