Dry ice is considered a "hazardous material" and must be disclosed upon checkin (the signs at each baggage acceptance point state as such.) I believe you can have 5.5 lbs in checked luggage.
There is a rather large fee for having it in checked luggage ($175), although I believe the same quantity applies for carryon, and no fee applies.
Don't "forget" to disclose this, as dry ice becomes gaseous CO2 upon sublimation, and if pets are in the cargo, this can become poisonous to them when the total quantity in the pit exceeds what is tolerable (I don't work on the ramp, so not sure if they must separate any quantity from the pets to a different pit, or if there is a total pit limitation.