If you go to check your email in the iPhone's Mail app only to see a blank -- either literally a blank space between messages or a place-holder that says (No Sender) (No Subject) -- you may have this bug. If you do we'll tell you how to work around it, after the break.
I experienced the blank email issue frequently under iOS 4.1, as does Andrew, but I haven't seen it (yet) under iOS 4.2. Ally has seen it over multiple iOS versions. (Ironically Ally captured this screenshot when she got a blank during an email exchange about blanks!) Whenever or wherever you have this problem, the only solution we've found so far is to restart (force quit) the Mail app:
Mail will auto-restart so you won't miss any messages, but when you go back in your blank(s) should be gone.
Let us know if you're experiencing this bug and, if so, whether or not this workaround works for you!