I HIGHLY doubt that Phelps is a habitual high-on. For someone to make a point of taking a picture of Phelps hitting a bong and then tweeting it (my views on twitter are already well-known), implies that is an infrequent activity. 2-3 times per month IMHO.
The pot-heads are trying to use this as, "See, pot makes you win Gold Medals." That's almost as absurd as "Pot cures cancer."