Originally Posted by NickW
As long as a UPS battery is compliant with IATA/ICAO special provision A67, it is classified as non-dangerous even though it may be a lead-acid cell.
My understanding is that the overwhelming majority of modern UPS cells are compliant (i.e. non-spillable even in case of a cracked/ruptured case). If in doubt, it should be simple enough to discover from the manufacturer.
My understanding is the same, mainly because manufacturers want to be able to ship the UPSs via standard methods without hazmat restrictions. As long as the battery is non-spillable (sealed, no ports for water), it should be OK. Read the label on the battery. It may even say "FAA approved" on it.
If you carry such a thing for whatever reasoin, you might get a copy of the actual regulations regarding batteries and carry it to show to the airline/screener if they give you a problem.
http://ash.faa.gov/docs/HAZMATByPassenger.pdf is a good start; it covers non-spillable batteries for wheelchairs and gives the disconnect requirement.