The mobile check-in is not just a way to tell your friends where you are via FourSquare or Facebook Places, it is a marketing opportunity. Southwest Airlines is combining Facebook Places and charitable giving to the Make-A-Wish Foundation to encourage travelers to check into Southwest when they get to the airport.

From now through Christmas, Southwest Airlines will make a $1 donation in the form of free travel credit to the Make-A-Wish Foundation, which grants wishes to children with life-threatening medical conditions. Often these wishes involve travel, an Southwest will donate up to $300,000 in travel credit through this promotion.

The promotion is a good example of cause marketing. It gives people an actual incentive to check in that is altruistic and makes them feel good. (Southwest is partnering with Microsoft to place photo-taking Santas at different airport terminals). Some of that good feeling might rub off on Southwest for making the connection, and it also has the added bonus of training people to use their mobile phones to check in at Southwest Airline locations. Next time they fly, they will probably check in again just to see what other geo-promotions Southwest is running.

Companies are still trying to figure out how to take advantage of geo-promotions. Turning check-ins into matching donations is certainly more compelling than trying to get people to check into billboards. What is the best check-in promotion you&'ve seen this holiday season