In its first press release since February, online music retailer eMusic announced on Monday that it had inked a deal with giant record label Sony Music Entertainment. eMusic, which offers music downloads by subscription (starting at 24 per month for $11.99), will have access to Sony’s immense music catalog — but only tunes that are two years old or older.

So, according to this agreement, subscribers can download “classic recordings from all Sony Music labels, such as Arista, Columbia, Epic, Jive, LaFace, Legacy Recordings and RCA, including artists such as Billy Joel, Bruce Springsteen, The Clash, Jeff Buckley, Johnny Cash, Leonard Cohen and OutKast.”  Because you can’t get enough of that fine music from the 1960’s and 1970’s (OutKast excepted).

All sarcasm aside, the partnership with Sony is a huge breakthrough for eMusic. Until now, the eMusic subscription service has been best known for its broad library of independent record labels, and work with Independent online IODA. Making a deal with Sony may open the door for eMusic, or another online music retailer, to forge relationships with the other major mainstream music companies, Warner Music, the Universal Music Group and EMI.

For Sony’s part, they have long wanted to dislodge Apple’s iTunes Music Store from its top spot in online digital music sales. Perhaps this is a small step in that direction (perhaps not). The other major music companies are working on alternate online music sales partnerships as well: YouTube and Universal Music Group are said to be in talks (and inviting the other three big players) to start a music service called Vevo.com, as well.

Even Microsoft is working doggedly on increasing their share of the online digital music market, with recent enhancements to its ZunePass music subscription service.

Certainly, competition would be great for consumers who buy content from iTunes, too. But Apple has the advantage of several years’ head start on these upstart competitors, and a clientele who are “well-trained” in browsing and buying through the iTunes site.

Terms of the eMusic/Sony deal were not disclosed.