PsyBlog recently highlighted Ten Studies About the Dark Side of Email.  One highlights why you should never negotiate (with clients on fees or opponents on settlement terms) via email:

Email negotiations often feel difficult, especially with people we don't know well. When Naquin et al. (2008) compared them with face-to-face negotiations, they found that people were less co-operative over email and even felt more justified in being less co-operative.

Part of the reason negotiations are difficult is that people tend to be more negative on email. For example, Kurtzberg et al. (2005) found that when people evaluated each other in performance appraisals using both pen-and-paper and email, they were consistently more negative about their colleagues when using email.

Yet another reason why, when the stakes are high, face-to-face wins the race.