I’m using PHP/nusoap:
$params = array("ident" => "N534UA", "howMany" = 15);
$result = $fa->call("FlightInfo", $params);
This gives me
no data available
But http://flightaware.com/live/flight/N534UA seems to work.
Other API calls work for me, like the example Metar. If I try InFlightInfo for that tail, I get an array of zero values, which I presume is because it’s sitting on the ground presently 
That tail number does its primary activity under the callsign “UAL720”, so if you change your query to use that instead it will succeed.
Not all of the FlightXML methods currently support lookups by tail number if the flight was using a callsign. InFlightInfo is one that does and its documentation notes “Codeshares and alternate idents are automatically searched”
Hmm, I was hoping to tie up tails to flights; how does the web site do it then?
Our website has access to more information than what is currently exposed via FlightXML.
You can use “AirlineFlightInfo” with the faFlightID of “UAL720-1325664767-airline-0046” and it will tell you that it is using tailnumber “N534UA”, however at this time there is no easy way to perform a FlightXML lookup directly by just tail number if it was actually operating under a call sign. We may add this functionality in the future though.
Ok, thanks.