You can't keep those satellite radio stocks down.

Though consumers are only now starting to discover the sublime satisfaction of paying for radio service, manufacturers, retailers and automobile industry partners have been lining up their support for XM Satellite Radio ( XMSR) and its recapitalized rival Sirius ( SIRI).

Now you can add investors to that growing list of fans. Shares of XM jumped $1.62, or 17%, to $11.32 Tuesday as manufacturing partner Delphi announced that the world's top retailer, Wal-Mart ( WMT), will carry its line of satellite car radios and boomboxes. The Washington, D.C.-based radio company has seen its stock more than double in the past three months to a 52-week high, putting it more than 500% above last fall's low. Following an analyst upgrade Tuesday, Sirius rose 30% to $1.20.

XM customers pay about $10 a month to receive more than 100 stations of radio programming. Bulls, who have long seen the car market as a captive audience for the companies' largely commercial-free broadcasts, are even further encouraged by the spread of satellite radio beyond the car.

But XM's speculative business plan and somewhat cultish product have attracted a number of skeptics. As such, the stock's rise lately also reflects the temporary retreat of some bears betting against the XM story. As at a number of speculative tech companies, XM's rise has been compounded by the rush of short-sellers cutting their losses and purchasing the stock at its higher price. But with short positions representing roughly 40% of the float, it's clear that not everyone has been won over to the XM case.

Radio Ga Ga
Buying the XM story

Notably, even some satellite radio bulls say the so-called short squeeze has pushed the stocks' value higher than their immediate sales prospects may warrant. XM continues to burn cash and at recent levels trades at a cool 50 times latest-year sales.

"These stocks are overvalued on a near-term basis and will probably stay volatile," says a New England-based hedge fund manager who is long Sirius. "This will cool down at some point, but the business and the growth is ramping up."

For now, the growth is enough, apparently.