The gap between twinks and non-twinks is wider at 59 compared to lower brackets, so that makes it a little harder to denote tiers. With that said, class tiers pretty much fall in line with the 39 and 49 brackets.
Disc priests are S-tier/god-tier. Even more so than 39s and 49s, Disc priests are juggernauts.
All healers are tier 1. That sounds like a sweeping statement, but their damage output and survivability make them that good. Resto shamans, resto druids, holy priests (that's right), mistweaver monks, holy paladins...all of them.
Non-tank warriors are tier 1. Stun city. Surprisingly, rogues seem tier 2 to me. They don't have the damage output they need. To be fair, I haven't seen any exceptional rogues yet.
Aff locks remain in tier 2, but don't underestimate them. When played well and when they get the team support they need, they can really pressure battles. Other lock specs seem stuck in tier 3.
Arcane mages hit hard, but I haven't played against a good enough one yet to convince me they surpass tier 2. Same for Fire and Frost at tier 3.
DKs have more potential than I originally gave them credit for, which brings them up to tier 2.
I haven't seen enough of the other classes and specs to pass judgment, but at best they're tier 2, and more likely tier 3.