Bloomberg.com: U.S.

Bloomberg.com: U.S.: "Sirius Satellite Radio Inc.'s agreement to buy rival XM Satellite Radio Holdings Inc. for $4.57 billion in stock is a bet that U.S. regulators will alter rules that bar the only two pay-radio companies from combining.Shares of XM may rise on the agreement, announced yesterday when U.S. markets were closed for a holiday. The terms value Washington-based XM at $17.02 a share,