I've got some good news and some bad news for XM Satellite Radio (Nasdaq: XMSR) and Sirius (Nasdaq: SIRI) investors.

The good news is that it may get a whole lot easier to get the merger between the two companies approved, after this morning's announcement that online radio service Last.fm will offer free streaming music from all four major labels and countless indies. A program is also in place to monetize uploads from unsigned artists.

The bad news? I just told you. Last.fm, bought by CBS (NYSE: CBS) in a $280 million deal last year, is providing on-demand delivery of its growing digital library for free.  

Naturally, this is also bad news for other companies selling digital tracks, like Apple (Nasdaq: AAPL), or music subscription services like Napster (Nasdaq: NAPS) and RealNetworks (Nasdaq: RNWK).

Models will be rattled. Sure, Napster has had a free lo-fi streaming option for Web-based users for some time now. And let's be clear that even the generosity of Last.fm has its limits. Once you stream the same song three times, you'll have to purchase it from affiliated retailers like Apple and Amazon.com (Nasdaq: AMZN).

You may find the restriction limiting -- perhaps even Zune-esque -- but the key to enjoying a music discovery site like Last.fm lies in exploring the millions of available tracks, without repeating the same tune over and over.

It's funny to see terrestrial radio lobbying against the pairing of XM and Sirius as a threat to their livelihood, when this "silent but deadly" killer is sneaking in through the Internet.

It doesn't take a lot of imagination to see where the music broadcasting industry is headed. Have you tried the FlyTunes widget, which lets iPhone users stream free Internet radio through their iPod-enabled wireless phones? Do you realize that more and more new cars are coming with jacks to plug in third-party gadgets? It won't be long before satellite and terrestrial foes are huddled together, shouting "bogeyman" at Web-beamed music programming.

I think that XM and Sirius are differentiated enough to earn their premium pricing. I can't say the same for free radio. It may be Last.fm's stand, but it's starting to feel more like the last stand for conventional one-way commercial music broadcasting.

For musical Foolishness:

XM Satellite Radio is a former Rule Breakers stock pick. Amazon.com is an active Stock Advisor recommendation. A free 30-day subscription will shed some light on either premium music service.

Longtime Fool contributor Rick Munarriz subscribes to both XM and Sirius. He does not own shares in any of the stocks mentioned above. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool's disclosure policy is always ready to rock.