Monday's New York Times had a good story about retired HP employees serving the company in a variety of ways. The work is unpaid, and that's fine by these folks -- a group ranging from still-very-vital college professors in their early '60's, right on up to guys in their '90s (some of whom retired wealthy through years of HP stock accumulation), who recall lunching with Mssrs. Hewlett and Packard themselves.
If it's not about the money, and knowing that many of these people have other (paying) things to do, what gives? Particularly when you read that the work can include Saturdays at Circuit City touting HP laptops to listless aisle crawlers.
It's about social connection. During our working lives, and I'd argue even more in retirement, we need to feel like we belong to something bigger than ourselves.