Citing "industry sources," Reuters claims that direct royalties may be part of the deal that finally got The Beatles on iTunes. According to Reuters, "superstar artists" (like The Beatles) typically get 20-25 percent of retail revenues, which would equate to between 18 to 22.5 cents per track sold on iTunes. Supposedly, The Beatles secured a deal with Apple that gives the surviving band members (and the estates of John and George) far higher royalties than that, though it's unclear just how much higher; Reuters's sources didn't put any firm numbers on the deal. Earlier reports indicated that it was primarily disputes between The Beatles and music label EMI that kept the band's music off iTunes for so long, and it's very likely that royalty distribution was at the heart of the matter.
To be honest, I'm not sure why it's taken this long for artists to get a bigger cut of digital sales. Under the old brick-and-mortar sales model, it seems like music labels would have had to pay out a lot more to market and distribute music than under a purely digital sales model. If The Beatles are getting a bigger slice of the profits from sales of their music, I see that as a good thing, and frankly I hope that becomes the rule rather than the exception.