Dry ice is limited to 2.5kg, must be in a vented container, and labeled as “DRY ICE” or “CARBON
DIOXIDE, SOLID.
http://www.faa.gov/about/office_org/...ersandcrew.pdf
Looks like United charges a $150 fee for having dry ice. Contrary to the older threads, wet ice is not allowed, so gel ice packs will probably be your best bet.