My rather recent experience is that you may actually bring a low amount of dry ice on board (either as carry on or check-in with proper shipping documents and stickers on the styrofoam box). However, make sure you first call the airline to fully discuss feasibility. If this is possible make sure you have proper signed documentation from the shipper (your institute officials), and full description of the cell line you intend to ship. Naturally, it should be used for research purpose only, non-hazaradous, non-toxic, non-infectious, not carrying, inoculated, or exposed to any disease agents, not originating from a facility where work on disease agents affecting livestock or avian species is conducted etc. If you can assure this, send all the documents to the airline well in advance and bring it with you to the airport. But phone again to make sure they have received it, and that it is OK before you arrive at the airport.