There are some active single-letter domains, e.g. http://x.org

See for details: http://en.wikipedia.org/wiki/Single-letter_second-level_doma...

whois says:

   Domain Name: A.COM
   Registrar: RESERVED-INTERNET ASSIGNED NUMBERS AUTHORITY
   Whois Server: whois.iana.org
   Referral URL: http://res-dom.iana.org
   Name Server: No nameserver
   Status: serverDeleteProhibited
   Status: serverTransferProhibited
   Status: serverUpdateProhibited
   Updated Date: 29-dec-2010
   Creation Date: 01-dec-1993
   Expiration Date: 08-dec-2011

No idea why they'd be reserved though.

Even back in the day it was hard to get a one letter one. That's why I had to settle for a two letter one: http://vm.com

iirc two characters was the original minimum length for a hostname.

Paypal has x.com