I'm pretty sure it's only going to be PvE and up until Leoric. PvP would make people not stop playing and they wouldn't be able to test the game on as many machines as possible (by giving out new keys) if people keep playing PvP forever.
I'm just imagining that's the reason. It wouldn't work with me though as I'd play PvE until the last day of the beta if I got a key =D
And I still don't know when, at what level, can we let our characters enters the arena.
If they're faithful to the PvP spirit, they'll let us do it from lvl 1 =] except people with gear bought from the Auction House (gold or rm) will probably roflstomp others. It would still be amazing.
If they have to put a lvl cap on the arena, anything above 5 would be outrageous <_