If you live in the United States, the AirCarrier Access Act which was passed in 1986 and amended in 2000 prohibits the refusal of transportating an individual with a disibility on the sole basis of this disibility. However - "The carrier may refuse transportation if there are no lifts, boarding chairs or other devices available which can be adapted to enplane the passenger."
The carrier may be liable for any damages that occur to your electric wheelchair during transportation. Unlike checked baggage the carrier can not have a limit for damages or loss of your electric wheelchair -- they must refund you for the full purchase price that you originally paid for your electric wheelchair.
While you are not required to give an airline advance notice that you will be traveling with a disability, it is recommended. However, you may be required to give a 48 notice if you need to transport your electric wheelchair in an airplane with less than 60 seats.
Be on time -- "Carriers may require passengers with electric wheelchairs to check in one hour before flight time." If you miss this time window, the carrier still must attempt to accomodate you unless this would delay the flight.
"When stowed in the cargo compartment, wheelchairs and other assistive devices must be given priority over cargo and baggage, and must be among the first items unloaded."
If your electric wheelchair must be disassembled in order to fit in the cabin or cargo hold then it also must be reassmbled before they return it to you. You have the right to give the carrier written instruction on the disassembly of the wheelchair.
Check out our page on renting an electric wheelchair while travelling if you would rather not travel with your wheelchair.
[source, more information - http://airconsumer.ost.dot.gov/publications/HorizonsPrintable.doc ]
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