Cutting through all the bs moralizing to answer the question posed, my opinion is $500 cash to a gate agent (who has operational upgrade powers) might yield an upgraded class of service 50% of the time. Aside from the embarrassment factor if denied, the security issue is crap.

Some other common bribes/tips (for the "this is stealing" crowd):

$50 to a host to get a nice talbe quickly at a busy restaurant
$50 to a hotel clerk asking for a nice room
$100 to the helicopter pilot to take an extra couple of spins around Sydney Harbour on a sightseeing flight
$20 to the valet to keep the car close
$100 to the appliance delivery guys to go ahead and attach the dishwasher hoses