I bring great news! I am almost positive that they'll be system read this quote from the press realease-
The first two Nick properties coming to life in brick form are "SpongeBob SquarePants" and the newest network hit, "Avatar: The Last Airbender," both rated in the network's top 10 for boys ages 6-11, according to A.C. Nielsen. Both lines will be available in August 2006.
As you see it says 6-11. The scale to system is... 6-11. Read this next quote-
LEGO Avatar: The Last Airbender offers two hero models with collectible minifigures and role play action to recreate popular environs of the animated series. Planned marketing support includes TV, Web content, LEGO Club, model tour, co-promotions and retail feature.
If they're both aimed at 6-11 year olds and also it confirms Avatar is minifig scale, I am sure SB will be to!
Also, if Duplo got Dora the Explorer and Bob the Builder I think we need 2 of our own aswell :D
This is my take on it ;)