Are terms-of-service enforceable?

The Electronic Frontier Foundation's Ed Bayley has written an excellent introductory white-paper on whether click-wrap, browse-wrap, and other online terms of service are enforceable:
In other words, it's not merely clicking the "I Agree" button that creates the legal contract. The issue turns on reasonable notice and opportunity to review--whether the placement of the terms and click-button afforded the user a reasonable opportunity to find and read the terms without much effort. In practice, the enforceability of each TOS implementation often falls on a sliding scale, depending on the degree of notice it provides the user. At one end, presentations that require the user, before clicking, to scroll to the bottom of a set of terms, or through an adjacent scroll box, guarantees the entirety of the TOS appears at least once, even if the user chooses to ignore it, and has been held to be enforceable. At the other end, by contrast, if a user must click on a hyperlink, or series of hyperlinks, to view the terms, the significance of clicking "I Agree" as showing assent diminishes, depending on the difficulty in actually finding the terms and whether a reasonable Internet User would have done so. Finally, in addition to the placement of terms, courts also consider the inclusion of conspicuous statements on websites that instruct users to read the TOS and inform them of the consequence of clicking "I Agree."...

Whereas courts have been willing to give clickwraps their blessing, attempts to legally bind users with browsewrap agreements have been more controversial. Unlike clickwrap agreements, browsewraps do not require a user to engage in any affirmative conduct, like clicking on a box, in order to show that they agree to a set of terms. Instead, websites with browsewrap agreements often purport to bind their users by passive conduct, unrelated to the TOS itself, like continuing to use the website or proceeding past its homepage.

The Clicks That Bind: Ways Users "Agree" to Online Terms of Service