As I understand it, offsides is defined by any part of the body other than the arms.
Here's the FIFA reading: http://www.fifa.com/mm/document/affederation/federation/laws_of_the_game...
In the defi nition of offside position, “nearer to his opponents’ goal
line” means that any part of his head, body or feet is nearer to his
opponents’ goal line than both the ball and the second last opponent.
The arms are not included in this defi nition.
scroll down to "decision 1" under law 11.
So, if my foot is slightly closer to the goal than yours is, then its offsides. There is no doubt that the Slovenia call is an embarassment, especially since the Ref. couldn't even explain why he made the call that he did. The Algeria call on the other hand was a legit close call. I've looked at the replays several times (from several angles) and I honestly can't say with any certainty that we (the USA) we're onside. Close offsides calls happen all the time in soccer. I honestly don't think that was a "bad" call at all, and I wouldn't infer bias. If the Slovenia call hadn't happened, nobody would be talking about the Algeria call today.