Deadlinehollywooddaily.com is reporting that the studio is not set on Josh Hartnett as Loki but could also be looking at Josh to star in the pic as Thor himself. It seems that the notion to cast an unknown is one the studio wants to at least pursue. Other actors are also being considered for the Son of Odin, they are listed below.
Charlie Hunnam (the British co-star of the F/X series Sons Of Anarchy)
Tom Hiddleston (award-winning British actor and RADA graduate who played Winston Churchill's son in HBO's The Gathering Storm)
Alexandar Skarsgard (Stellan's son who has appeared in the HBO Iraq War miniseries Generation Kill and vampire drama True Blood, and who's definitely visually right for the role)
Liam Hemsworth (offered a significant role in The Expendables after Sly Stallone saw his tape)
Joel Kinnaman (some Swedish-American dude)
Another tidbit of information leaked in the same article is that Natalie Portman is in the lead for the female lead role.