Actually... sponsors don't actually buy cycling teams. The teams are independent sporting orgaizations [groups sportifs] registered with the UCI which sign contracts with sponsors for funding.
The company that actually pays Lance ARmstrong's salary is a sports marketing firm called Tailwind Sports. When the USPS sponsorship contract with Tailwind expires in 2004, there's nothing stopping Tailwind from signing up Federal Express, or whoever, as a title sponsor.
I'm sure the Post Office has a fair bit of say in how the team is run, but only as an important sponsor.