I have heard of a few guys w/ over heating issues over on an F150 forum I frequent, but it does not seem to be widespread. The ones who complain (and I don't use that in a derogatory sense) about it say it happens on long slow climbs up dirt roads and so forth.
I have also heard some problems with the fan relay. Apparently there are 3 fan speeds, with a couple relays that make the highest speed kick in. A problem with the relay won't manifest itself until you really need the extra cooling. I know when my max fan kicks on, it sounds like an airplane taking off.
FWIW, I haven't had any overheating issues with mine.