It was purchased before 1993 by Weinstein & DePaolis, and subsequently sold to Paypal (or the company was bought out). In 1993 IANA reserved all remaining single letter second-level domains, and grandfathered the ones already issued. Other functional, corporate examples domains are t.co (Twitter) and q.com (Qwest).
I hate to cite Wikipedia as a source, but it has an acceptable article on Single-letter second-level domains:
On December 1, 1993, the Internet Assigned Numbers Authority (IANA) explicitly reserved the remaining single-letter and single-digit domain names. The few domains that were already assigned were grandfathered in and continued to exist.
The assigned domains in this group are the following:
i.net INet Solutions Ltd Future Media Architects
q.com JG Qwest
q.net Privately owned Privately owned
x.com Weinstein & DePaolis PayPal
x.org The Open Group X.Org Foundation
z.com HomePage.com Nissan Motors
As of April 2009 only three domains, i.net, x.com and x.org host a web site. q.com is active but redirects to qwest.com.