Wow, what a big surprise. For a binding contract the contracting parties must be aware of the contract and it's conditions otherwise there is no contract. Who would have thought this.
What surprise comes next? Maybe that laws, especially privacy and consumer protection laws cannot be waived by terms of service? Or that there are different countries with different laws, and clauses that are commonly used in the US are often null and void in other countries especially EU countries because of other laws?
However, it also means the links to LEGAL or TOS on bottom of every page on a site don't mean squat and are probably unenforceable unless this is overturned.
Don't count on this to be overturned. This is basic contract law and nothing new. It's like Software TOS that have repeatedly ruled to be invalid because they were either only accessible after purchase or there was no requirement to view the license during download or even installation.
And actually it is of not much consequence either because usually you only need TOS once a user starts to interact with a website - for example signs up to a forum or a newsletter. And it's no big deal to add a box with the TOS during the signup process.
Zappos could have easily avoided this, by including the TOS in the signup process. However even then I doubt that this arbitraton clause would be valid. In many jurisdictions clauses in TOS that are deemed to be unusual or surprising are not enforcable. Waiving your right to go to court might be seen as such a clause in some states and countries.