Back in 1993, IANA set aside the registration of single-digit domain names (six were floating around – IdeaLab used to own Z.COM, for example). Now ICANN is supposed to discuss whether such names should be released. Presumably some of these names would be quite valuable.
Some companies are making (possibly) premptory trademark filings. Yahoo has filed a trademark application for Y.COM. Overstock has filed for O.COM. A company named GQXZP2 (represented by Bracewell & Guiliani) has filed several ‘single’ applications, including A.COM.