HS-on will definitely go on until September 1st, 2012. Throughout the years, the rotation pattern remained the same. A rotation would happen on the September 1st of a year, and this would mark the beginning of a new format. That format would go on until September 1st of the year after, where another rotation on the same day occurred, starting another new format, and from there, the process would repeat.
The only reason why they made a rotation on July 1st instead of September 1st was because of one card: Sableye. The new rule changes made when Black and White was released turned Sableye into an overly broken card. It could now abuse Trainers and Supporters on the starting turn to the point it could pull off the donk super easily compared to the MD-CoL format. This lead to the creation of Sabledonk, which focused on that exact strategy. In the end, however, it was really Sableye in combination with Trainers and Supporters on the first turn that was problem, as players started to discover ways to actually pull off easy donks with Sableye in other decks: those decks being Luxchomp and Gyarados. Such a situation would cause nationals and worlds to become extremely disappointing because many players wouldn't be able to experience full games, and that would just take the fun out of the game. Plus, when BW was released in Japan, Sableye was already rotated, so it wasn't much of concern compared to what happened internationally. Outside of Japan, The rule changes caused huge shock on how donks could be pulled off more easily than before; so, in the end, to make the game fun for everyone for nationals and worlds, they chose to make the format HS-on with a rotation happening on July 1st, and it is already evident that the format is much more balanced compared to MD-on.
As for the sets that will be rotated in the next rotation, It'll most likely be all of the remaining Generation 4 sets: Those sets being HS-CoL. This would leave the format BW-on. Why would they do this? Well, what has usually happened is that whenever a new generation set is released, a bunch of sets are rotated out to make way for the new generation, leaving only a few sets of the past generation behind. A good example of this was back when DP was released to the TCG. When it was released, it caused the next rotation to rotate out 5 of the 9 Generation 3 sets remaining: Those being Deoxys-Legend Maker. This left behind only 4 sets from Generation 3 (those being Holon Phantoms-Power Keepers) in the next year, and then the year after that, all of those 4 remaining Generation 3 sets were rotated out, leaving behind a DP-on format for the next year. With this in mind, one can expect a similar thing to happen with Generation 5's release; heck, it is already beginning to happen. With BW's release, All of the remaining Diamond, Pearl, and Platinum sets were rotated to make way for the newest generation, leaving behind only 5 sets of Generation 4: Those being the HS sets. By going the usual pattern, we should expect all of the HS sets to be rotated on September 1st, 2012, thus leaving a BW-on format for the 2012-2013 year.
If this is getting a little bit confusing, let me sum up my point. This is a new generation of Pokemon. Remnants of Generation 4 won't last forever (modified TCG wise). Play! Pokemon wants us to move on to Generation 5, so one can expect the next rotation to rotate out all of the remaining Generation 4 sets. Also, one can expect the next rotation to happen on September 1st next year because at the rate that this is going, the new format is not going to get very imbalanced as opposed to the days of DP.
