That's explicitly illegal to do in the United States. See: Magnuson–Moss Warranty Act - WikipediaMy guess is that Apple will invalidate the warranty if there's 3rd party service. Makes sense from their perspective, considering safety issues.
FTC's statement regarding section 108 of the act:
https://web.archive.org/web/20100528040418/http://www.ftc.gov/bcp/edu/pubs/business/adv/bus01.shtm#Magnuson-Moss:"Tie-In Sales" Provisions
Generally, tie-in sales provisions are not allowed. Such a provision would require a purchaser of the warranted product to buy an item or service from a particular company to use with the warranted product in order to be eligible to receive a remedy under the warranty. The following are examples of prohibited tie-in sales provisions.
In order to keep your new Plenum Brand Vacuum Cleaner warranty in effect, you must use genuine Plenum Brand Filter Bags. Failure to have scheduled maintenance performed, at your expense, by the Great American Maintenance Company, Inc., voids this warranty.