What's the deal with #Rule40 that numerous track & field athletes are tweeting about?

With the track & field portion of the London Olympics starting on Friday, here's a smattering of links and stories in the run-up to the start of competition:

RULE 40

If you've been on Twitter, you've noticed that a bunch of American track & field athletes have tweeted to their followers a message that looks like this:

"@AthletesHandle I am honored to be an Olympian, but #rule40 #