The Beatles are one of a few big bands who have yet to sell their wares online. The only other major bands that we could think of (and we're probably missing a few) that aren't on iTunes are Tool, Radiohead (!), AC/DC (!!), and the mighty Led Zeppelin. With the Beatles hopping on board, that'll narrow that list and hopefully make the others finally give in.
You would think with all the gobloads of ways the Beatles have been marketing their stuff that putting their music on iTunes would be a no brainer, but Apple and the Beatles have been fighting for years over the word "Apple." Mainly in that Apple also happens to be the long lost name of the Beatles record company (man, did we love the Apple logo they put on the lps- one side would be the outside of the apple, the other the inside of the apple, cut in half). After Apple (Computer) finally beat Apple (record company) in the greatly named Apple vs. Apple dispute (if only Gwyenth would go involved in all this so we could have an Apple vs. Apple vs. Apple dispute), tensions have cooled between the two camps, making this whole wonderful thing possible.
It also goes without saying that all of this kind of coincides with the release of the Beatles "Love" cd, which is a bunch of Beatles tunes re-done and resmastered (kind of like the infamous Grey Album except not illegal and without Jay-Z) and Apple is about to go to music war with Microsoft over mp3 players.
Best part of this? Waiting for the infamous iPod silhouette commercials all set to Beatles tunes.