Only about 6 single letter and single digit domain names have been registered by a few lucky or ###### insiders. ICANN reserved all single letter and single digit second-level domain names in the TLDS .COM .NET .ORG back in 1993. This was done in case the registries for these domains became overloaded (smells fishy to me). The 6 domains that were already registered were not revoked and were exempt from this ruling and allowed to stay in use.

Domain	1993 Owner	Current Owner
i.net	INet Solutions Ltd	INet Corporation
q.com	JG	Qwest Communications
q.net	Q Net	“Q Networks”
x.com	Weinstein & DePaolis	PAYPAL
x.org	The Open Group	The Open Group
z.com	HomePage.com	NISSAN

Source: Wikipedia