Satellite radio bears had to be running low on ammo after Sirius XM Radio (NASDAQ:SIRI) unleashed some upbeat fourth-quarter metrics this morning.

Remember the subscriber defections that haunted Sirius XM during the first half of 2008? Well, that's history. The company closed its latest quarter with 257,000 more subscribers than when it started. This comes after tacking on 102,295 net subs during the third quarter. It's true that Sirius XM closed out the year with fewer subscribers – second-half gains didn't offset first-half net account cancellations -- but the trend is unmistakable.

Another cynic-silencing metric is the company's cash flow situation. Sirius XM now expects to report more than $100 million of free cash flow in 2009, a refreshingly welcome improvement over the $552 million negative free cash flow it reported a year earlier on a pro forma basis.

My favorite metric, though, isn't the subscriber or free cash flow gauges being parroted by the media. The most encouraging sign of Sirius XM's relevance comes from its conversion rate. Buyers of new -- and in a growing number of cases, used -- cars with satellite receivers typically receive free trials to check out the service. The percentage of users opting to pay for Sirius or XM service after the free trials had been steadily declining during the recession, but is finally bouncing back. This isn't just good news for Sirius XM. It's also good news for automakers, as Ford Motor (NYSE:F), General Motors, and other car manufacturers get a piece of the action when trial members sign up.

During the quarter, 46.4% of trial listeners became paying customers. It's a drop from the 46.8% clip posted during the third quarter, but it's comfortably ahead of the 44.2% conversion rate posted a year earlier.

Conversions were better before the economy soured, but this is still encouraging. It suggests that the drop in Sirius XM's conversion rate was more the handiwork of the recession than the emergence of new technologies. Apple (NASDAQ:AAPL), Research In Motion (NASDAQ:RIMM), and Palm (NASDAQ:PALM) smartphone owners have easy access to Pandora and other streaming content, but satellite radio is still filling a void in aural entertainment.

Connectivity and streaming content may be problems down the line, but bears may have to hibernate for several more seasons -- if not years -- before those pessimistic arguments stick. Sirius XM is rolling right now.

Do you think Sirius XM will gain or shed subscribers in 2010? Share your thoughts in the comments box below.

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Longtime Fool contributor Rick Munarriz is a subscriber to both Sirius and XM. He does not own shares in any of the companies in this story. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.