I'm going to put the smart money on the 1-year anniversary date of the Hero, or maybe a week or three later. Why? People who bought Hero phones are the most likely customers to buy a Supersonic, many of them bought a Hero the weekend it came out, and none of them are going to buy a new phone less than a month before their 1-year anniversary date & get raped on the price, even if god himself were to part the skies, thank Linus Torvalds & Google, and encourage Sprint customers to buy them right away. Sprint's going to want good first-week sales, so they're going to make sure they time it so customers likely to buy a Supersonic right away will, in fact, be able to do so without being held back by rebate eligibility dates.
By the way, if you take advantage of the 1-year upgrade, does sprint tack 2 years onto your current contract's expiration date, or does it just extend your current contract so that its new ending date is two years after the purchase date of the new phone? It would really suck to get 4 or 5 new annual-upgrade phones, then end up discovering you've basically sold your soul to Sprint for the rest of your life (because you'd still need new phones, and would have to extend your contract more each time).