[daten]

In my experience this works every time at Dulles international (IAD) but never works at Hartsfield-Jackson Atlanta International (ATL). At ATL you get sent to the back of the normal line.

[jrockway]

The hack is to print FIRST on your boarding pass. The person checking whether or not you are allowed in the priority line does not have access to the reservation system to check that you are actually in first class. You keep your real boarding pass in your pocket and show that to the TSA and gate agent.

Note: untested. I get to use the first-class lines anyway :P

[eli]

And, hey, worst case you get detained by security and charged with a felony.

[jrockway]

An easy one to talk yourself out of, though: "Oh, something must have happened to my upgrade since I printed my boarding pass last night."

The 1950s-era reservation system isn't going to refute your story.