Here's a trick I found quite by accident. You can jump directly to a list of your draft emails in Mail for iPhone by pressing and holding the compose button:
Just hold your finger down and a list of unfinished drafts pops up. Tap any one to jump right to it. I just happened to press and hold the compose button and there it was. Excellent.
As with so many other iOS tips, this also works in Mail on the iPad and iPod touch as well.