Both XM Satellite Radio (XMSR) and Sirius Satellite Radio (SIRI) are trading sharply higher today on little news. The shares could be getting a boost, though, from a post today on Seeking Alpha, by investor Richard Dorfman. (The post has been picked up by Yahoo Finance.) Dorfman operates Richard Alan & Co., which describes itself as “a financial advisory and investment company.”

Dorfman’s writes that he anticipates the proposed merger of the two companies will eventually be completed, and that “the upside potential is substantial, with the resulting company well-positioned to become a powerhouse provider of audio entertainment.” Dorfman writes that he recently bought shares of Sirius for his own account. “I chose it over XM because I’m a big fan of Karmazin and I like the fact that Sirius achieved positive free cash flow in the fourth quarter of 2006 – two attributes which I believe make it better positioned than XM in the event the merger is quashed by the government,” he writes.

XM today is up 65 cents at $12.37; Sirius is up 12 cents at $3.10.

Update: As reader Bob points out in the comment section below, there’s also a report on TheFlyOnTheWall.com which says that XM was moving higher on “unconfirmed takeover speculation…There is chatter circulating on a competing bid for the company.” Not sure why Sirius shares would go up on a rumor that someone is bidding for XM, but whatever. Now you know.