marshrat is correct, freshwater catfish do not have venom or poison in or on their fins
however
salt water catfish known as hard heads, do infact have venom in their spines
all catfish have sharp spine fins, but the bigger and older they get the more dull and rounded off they get.
i still wouldnt advise pushing your luck either way tho, a spine in the hand will ruin your day
2thebream, i disagree, Venom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite, sting or other sharp body feature.
http://en.wikipedia.org/wiki/Venom
venom is an umbrella term that goes from everything from snake bites and bee stings, to gila monster and komodo dragon saliva, to sting ray barbs, their not all the same venom, but they are all just strands of protein, hell even the lowly humble fire ant has venom
and scientists have done studies and found venomous catfish, as sited here
http://www.sciencedaily.com/releases/2009/12/091203222139.htm
when they get bigger ther fins arnt so sharp. dont be fooled theyll still getcha.