This just in -- General Motors has told customers that in the future they must only buy gasoline from Exxon stations. GM will get a share of Exxon's revenues as part of the deal.The car and truck giant has fitted sensors in its new vehicles to make sure customers don't cheat. Where it finds "unapproved" gasoline, GM is invalidating the vehicle's warranty and sending out agents to cripple the cars' engines, rendering them useless. The agents also let themselves into GM vehicles to remove unapproved accessories. "We are protecting the integrity of the user experience," said a spokesman. "Customers are only allowed to have a radar detector, or a new seat cover, when we say so." Pure fiction? Obviously. GM would never behave in such a totalitarian or bone-headed way. There would be an outcry if it did. But the scenario above isn't far-fetched. Just ask anyone who bought an iPhone. This is exactly what Apple ( AAPL) has just done. In case you missed it, the company used its latest software upgrade to cripple every iPhone that had been opened up to unapproved networks, and stripped out "unauthorized" third-party applications. Apple's message to its customers: You will use the network we tell you to use. You will use the applications we tell you to use. Memo to Apple: You guys are kidding, right?