I've heard a rumour that on most (American) airliners, you can always ask the stewardess for a free upgrade to businessclass seats (if available) on transatlantic flights, since they don't mind where you sit once you're in the air.
Is this true or just a wild fantasy?
Dream on!
On Continental, the dividing curtains between Business First and Coach are firmly closed once airborne and woe betide a coach passenger if he/she dares to try to use one of the BF toilets!
There was a hoo-hah a few years ago where a couple of Continental Flight Attendants were sacked for seating friends on "Buddy" tickets in the BF cabin after take-off (one of CO's Board members just happened to be on the flight, incognito,) So now, unless you have a paid for BF seat, or you are a bona fide upgrade holding a BF boarding pass, it's not going to happen.
For a while, I think American Airlines (like British Airways) used to upgrade on board for a small fee taken by the senior crew member (BA was £250 from Economy to Club) but that has been stopped. American Airlines might still offer an upgrade for a fee at check-in if the Business Class cabin is pretty empty - not sure though.
The pricing of the premium cabins has reduced with the recession so empty premium cabins are becoming less likely.