The details are mostly what we expected: Extending your lockout will let it roll over into the next lockout period, consuming your instance ID for that day (in the case of heroics) or week (in the case of raids). You can extend your lockout more than once, probably indefinitely, but Eyonix wasn't specific on how many times you can do it in a row. I assume indefinitely. The extension is also on a per-person basis, meaning if you PUG someone into your 25man run, they can't screw you by extending their lockout. That also means it's not a raid leader decision. At least, not mechanically. It's up to the individual to click that extension button.
The extension system really wouldn't make sense any other way, so this is all pretty expected. While your raid leader will certainly have some sway over what your raid group does from week to week when in an progression guild, it shouldn't surprise anybody that Blizzard hasn't put your raid IDs solely in the hands of the raid leaders. That would be abused in the most epic way possible. Get a jerk of a raid leader in a PUG, kiss your Archavon ID goodbye for the rest of eternity because the guy keeps pushing the extend button? Yeah, no thanks. I think we're all glad that the decision is made by the individuals.
I eagerly await the inevitable, "Oooops, I 'accidentally' extended my raid timer" excuses from raiders that want a week off and don't have the guts to be honest about it. Those sorts of things are the best!
Patch 3.2 will bring about a new 5, 10, and 25 man instance to WoW, and usher in a new 40-man battleground called the Isle of Conquest. WoW.com will have you covered every step of the way, from extensive PTR coverage through the official live release. Check out WoW.com's Guide to Patch 3.2 for all the latest!