An EULA is like any other pseudo-legal document. Some parts of it will be legally binding, others won't be. For instance, extreme sports companies often have a disclaimer for liability over death and serious injury. In reality it's unenforceable, regardless of whether you've agreed to it.
I'd imagine Psytar's claim will firstly be that Apple is monopolistic (it's arguable - they're preventing you running their software on anything but their hardware, and can set prices accordingly. You could make a similar argument about Itunes). I'm not that clued up on US competition law, but on the surface they'd actually have a reasonable claim there.
Secondly, they'll almost certainly argue that even if they're not anti-competitive, the EULA is unenforceable.
It'd definitely be interesting to head Apple arguing that they're not anti-competitive, given the circumstances...