Overstock.com is trying to make their case of getting o.com if indeed they do release single letter domain names.
Much of the same case for U Magazine (
http://www.colleges.com/Umagazine/). You'll see that they are trying to brand themselves as "U.com"
Just something to think about. You might want to search the
USPTO Trademark Search to see how many of the single letter domain names have been trademarked.
As of right now, the search looks like it is temporarily unavailable, so you might have to try it later. But, just to see if it works, try it once or twice now.
Hope that is the information you are looking for.
-Steve