A contract can be enforceable even in email. Regarding electronic signatures, they can be used. Here is a good article at About.com regarding the Electronic Signatures Act, signed by former President Clinton last year. The Act "imposes a uniform national standard permitting, but not requiring, the use of electronic or digital signatures to enter into contracts and other legal agreements. An electronic signature would be as legally valid and binding as a written signature." Note that there are exceptions.
About: http://lawyers.about.com/careers/lawyers/library/weekly/aa061900a.htm
Copy of the Act: http://thomas.loc.gov/cgi-bin/query/z?c106:S.761.ENR: