I can think of a couple of examples from the sites I use regularly - ba.com and tt.com - which take opposite approaches:
British Airways redirects ba.com to www.britishairways.com but since ba.com is so short I don't even bother to bookmark either of them as typing ba.com in the address bar gets me there just as fast. I wonder what % of visitors to British Airways type [www.]britishairways.com and what % type [www.]ba.com? In the UK if you say 'BA' most people would know what you meant, but outside the UK I'm not so sure ... perhaps they went with with britishairways.com with an eye on their worldwide audience?
The second (rather obscure!) site is a regional Austrian newspaper called the 'Tiroler Tageszeitung' (roughly translates as 'Tyrolean Daily Paper') which I read regularly during the skiing season! Their site is www.tt.com and although they own tirolertageszeitung.com and tirolertageszeitung.at the redirects don't even work properly ... which gives us a fairly big clue what % of their readers use tt.com rather than the longer versions :-) The entire brand is built around 'TT' - and these two letters are used in all their print ads which are plastered round Innsbruck.
My bookmarks list has got so long that I can never find anything and find it quicker to search or type URLs in directly... anyone else have this issue?