stockquotes.mobi sold for 27K at auction (11 letters) - so noone really knows yet - it would make sense that the shorter the better, but imo it is more about the memorability factor - once entered (or spoken) they can be bookmarked or indexed.
id take ~ ringtones.mobi, dictionary.mobi, transportation.mobi, education.mobi, creditcards.mobi, reservations.mobi, hotelrooms.mobi, restaurants.mobi etc. etc. etc.
all the above are over 9 letters, but who cares?