I myself have had problems with this... there are alot of threads discussing solutions.. but to save you some time this is what I have come up with...

get the latch replaced... the new mechanism is redesigned and will cure this problem... if you dont have the time or $ to get it fixed... you can do what I have been doing until I get the X to my indy... I have found that if you can park the X in a garage or keep it warm it wont freeze up... when I have been unable to do this and must leave it out overnight or traveling cross country in bad weather... I have found two ways that have worked for me...

the least recommended thing is I start my car about an hour before I need to leave.. and I crank the heater.. and this thaws out the lock..this has worked once or twice... (I live in a military community and am not to worried about theft) however this is highly not recommended.. for obvious reasons..

also I have found that I can spray lock defroster in the exterior key hole and also on the door latch and this has worked a time or two...

in summary...

Replace the latch
De-icer
crank the heater for a long period..

hope this helps...