Coming back from a recent trip to Shanghai, I was "involuntarily" upgraded by Continental from my assigned Coach seat to a seat in BusinessFirst class. This upgrade would have cost $500 plus 25,000 miles if I'd scheduled it. Instead, I got it for free.
The reason? Coach was oversold.
I wish I could say that getting one of these upgrades was a matter of asking. The real key, though, is elite status.
Like most of the legacy airlines, Continental doesn't give complimentary upgrades to anyone on long-haul international segments. If there aren't enough seats in Coach for everyone who checks in, though, they'll start upgrading people in order of seniority based on elite status until they've taken care of the overflow.
This scenario happens surprisingly often on U.S. carriers operating flights from overseas destinations (especially Asia) back to the United States. It's quite common for flights that have little booking on their outbound routes to be packed coming home.
Of course, there's a downside: they'll only upgrade as few people as needed to get rid of the overbooking. In my case, that left my wife in Coach while I was put in BusinessFirst--she did insist that I take the seat, you understand--because that left Coach completely full.
I'm lobbying Continental to allow people in this situation to buy discounted upgrades for their travel companions at the gate; we'll see what comes of that.