The 2012 Arena Pass is now open for registration but will not be available to play on until later in April.

They added a FAQ (Frequently Asked Questions) list as some of the wording in their previous post was a bit vague.

WHAT IS THE 2012 WORLD OF WARCRAFT ARENA PASS?
The 2012 World of Warcraft Arena Pass is a service based on the Arena System in World of Warcraft. All teams participating in the Arena Pass will be competing on regional Arena Pass realms — separate from the live realms — for prestige and glory. In addition, there is a registration fee to join. This specialized format provides a means for high-level players to face off in an intensely competitive environment that emphasizes strategy, skill, and execution over all other elements of play.
WHAT IS THE FORMAT FOR THIS YEAR’S ARENA PASS?
Participating players will gain access to our dedicated Arena Pass realms and be able to create up to three level-85 characters per registered account. This special realm will allow players to fully customize characters with some of the best items available in the game. We will continue to use the 3v3 Arena format for this years’ service. Please note that the standard 2v2, 3v3, and 5v5 Arena season games will continue as normal on the live realms and will not be interrupted by this service.
Also, for the first time ever, participants in this year’s service will receive free entry into a level-90 phase of the Arena Pass realms once Mists of Pandaria is released. These realms will feature epic gear from the expansion.

Level-90 character templates will now be available and outfitted with gear from the Mists of Pandaria expansion, and vendors will offer additional options should the templates not fit your play style.
No additional awards will be made available at this time. The second phase of the Arena Pass is to provide a fun sandbox environment to test your skills in Mists of Pandaria.

Sound good, looks like Blizzard want to balance Mists of Pandaria sooner than 3 patches are already dispatched in the expansion!

TL;DR: Arena Pass will be in Season 9 gear but in the current patch, starting in roughly 2 weeks. Practise period lasts for 4 weeks, then rated period is 4 weeks. The Mists of Pandaria Arena Pass will purely be a sandbox for when MoP is released to test PvP in the new expansion (to give feedback to blizzard I’m guessing).