I've flown on southwest, AA, Alaska, Northwest, America West and USAir domestically. with my autistic DD, as well as internationally on USAir & Virgin. I would wholeheartedly second the DVD player suggestion, but I can honestly say, there wasn't a single airline that turned me down for preboard when I told them my child was autistic.
What really worked for me is that my daughter loves Disney everything--it is her special interest. We bought cheap playset toys with Disney Characters at the local KB or Toys R Us, and she would play for hours with no problems. If your child has some special interest you could accommodate (Thomas the Tank Engine, Barbie, etc.) this might work for you.
One thing I would definitely add--you are wise to take your daughter on a plane while she is young! My daughter's now a seasoned 11 year old traveler, and knows the gate procedure better than most adults. Her friends who have never been on a plane have a lot more trouble when their first flight is when they're 11 or 12.
She thinks of it as a more or less expected experience.
Since this is the first flight, if you can, I would go online and get a map to the airport--find out what restaurants and facilities are/will be there, and where they are. Questions about "where are we eating", etc. can be answered much more easily this way.