We've now flown with tandems four times, most recently two weeks ago. I thought I'd write down what we've done.

1) Greenspeed tandem trike, S+S couplings, back from Australia. S+S worked well - wheels in one rucksack, most of frame in other, seats as extra pieces. Glad they didn't weigh it too carefully though. Minor ding in kingpin needed reaming out on arrival.

2) Road tandem, Easyjet to Majorca
They ask for them to be boxed, so we got a pair of cardboard boxes from the bike shop and put them together. Quite a lot of disassembly. Carried on top of hire car, strapped down on some pipe insulation and with the straps running through the windows - not that great.

3) MTB tandem, Virgin Atlantic to Las Vegas
Virgin accept bagged, so got two of the CTC plastic bags and lots of parcel tape. Pedals off, saddles dropped, suspension deflated (to make it smaller), tyres deflated, handlebars off, rear mech off. Some pipe lagging round the discs. Jiffy bag for the removed bits, taped to frame along with heavier tools.
Worked well. I got moved on from the Las Vegas luggage area where I was putting it together - took a bit longer than I'd hoped - but I manged to ride it from the airport with everything. (Las Vegas is very small - 4 miles or so from airport to hotel on the strip).
TSA objected to my empty petrol bottles on the frame on the return trip, and sliced the plastic to remove them and didn't tape it up afterwards :-( (after a little bit of discussion with a man who couldn't come up with any ideas at all, I went and gave them another wash and brought them in as hand luggage - in the open. They said "What's that" "It's an empty petrol bottle, freshly washed out" "Ok" - so barely a flutter. The soapy smell probably helped, and obviously the bottles had their tops off.

4) Road tandem, Thomas Cook to Tenerife
Again, they accept bagged. You have to pay for a tandem, but that at least means no scope for rejection. Front wheel off, tyres deflated bars off, pedals off, rear mech off, brake levers tied to bars, gear levers removed and taped to bars (cable in place). Jiffy bag for bits again. This time we only needed one of the CTC bags, and lots of tape (partly to do up the TSA's hole...)
Must remember to remove skewer from front wheel when carrying - ours was bent. Fortunately I'd put another hub in the fork to protect it so had a spare. Lack of front wheel also means timing chainrings are a little more open to damage - I had to bend a tooth back.
The small packing meant the bike also went fairly easily inside our hire car - a citroen C3. Which made life nice and easy.
No legroom on plane :-(