Yeah, its hard to say there is ONE dump stat. It really depends heavily on your build.
CON is a good stat due to extra HS and base hit points, but the difference between a 10 and a 14 isn't exactly HUGE. Add in the fact that there's exactly ONE skill based on CON and it pairs with STR for FORT, which is a key stat for a lot of classes, and it really balances out.
DEX and STR obviously are heavy favorites for weapon using classes, but that covers less than half the classes out there, and even Martial classes always have some heavy favorites for secondary and tertiary stats.
I've seen characters 'dump' pretty much every stat, but there's always some reason most any character would LIKE to have a higher value even in their 'dump' stat. You just have to accept that you will have somewhat of a weakness, or else try to go with a more even array and accept that you're not quite the best possible ever at too many things (though an INT primary character can do pretty well spreading his other stats around a lot).