The 0.9.12qq patch will be deployed on Tuesday, October 9 (NZ time). When it is deployed, all default and hardcore characters (and their items) will be moved to the legacy and hardcore legacy leagues, respectively. The resistance penalties that we have been testing will be enabled on all leagues (at -20%/-40%/-60%).
Legacy migrations are performed occasionally during Path of Exile's Closed Beta in order to move old/unbalanced items out of the default league so that we can get good quality feedback from players' activity. The last one was five months ago, when we deployed 0.9.9. Recently we've been running one-week hardcore events to get balance feedback, but other users have expressed interest in new (especially non-hardcore) leagues to play in.
The new default and hardcore leagues will probably last until the start of Open Beta. It is possible that we will do another migration before then, but that's looking unlikely at this point. The new leagues will have prizes, which will be announced before the leagues start on Tuesday.
In addition, it's worth mentioning that once we add PvP arenas in the 0.9.13 patch, there will be a level 28 PvP tier (as well as the obvious unrestricted category). This means that you could start planning and making your level 28 PvP characters in advance in these new leagues if you're interested in that type of PvP mode. Please note that there will probably be a restriction that means that only characters who have never entered Cruel difficulty are eligible for the level 28 PvP tier. This is to prevent people feeling that they have to rush their character through all the higher difficulties to get the full set of skill point and bandit request rewards. Eventually we'll be adding other PvP tiers, but this is the first one that we're sure of.
It is still our plan to enter Open Beta during 2012. Once the game is in Open Beta and we have done our final wipe, we won't be doing legacy migrations any more. The default leagues will exist forever, but we'll be running special event leagues of various durations (from a few hours to a few months). Once those leagues end, their characters move back to the default leagues.
I understand that while people have been clamouring for a fresh start, many players are going to be disrupted by Tuesday's legacy migration. You're welcome to continue playing your old characters in the legacy league (and can maybe trade for some old overpowered items to have fun with). It's important that we do this week's migration during Closed Beta, so that we can get vital balance feedback before entering Open Beta.
Best of luck with the new ladders. I'll be playing.