If you limit it to 1 litre or less, you shouldn't have to pay any import duty.

Do not try to bring it in your carry-on luggage, pack it into the luggage that you check into the cargo hold. This is because Homeland Security has rules about how much liduid you can carry with you and they are severely limiting.

In order to help avoid leakage, pack the bottle so that it is upright when you carry your luggage.

I've had high-thujone absinthe delivered to my house, don't see why they'd stop you from bringing in small amounts for personal use. I believe that it is not illegal to own, just illegal to sell. If it is sold in Canada, you should be able to give it away at Pennsic.

I hope you have a blast.