According to the article:

  Using Clickthrough Agreements.  Zappos had an easy way to 
  form a clickthrough agreement.  As shoppers are checking
  out of the store with their shopping cart, Zappos could
  say "By clicking the 'purchase' button, you agree to the
  Zappos terms of use" with a link to the document.  It's as
  easy as that.  No custom coding, no interstitial web
  pages, no real risk of abandoned shopping carts.

Considering that the author is a professor of Law at Santa Clara University, I would think that the TOS in the login dialog would hold water in court.

http://www.ericgoldman.org/biography.html