Paul McCartney made the jump to iTunes, and now John Lennon's solo catalogue has followed suit. Three members of The Beatles now have their solo music catalogues on the United States iTunes Store, even if The Beatles themselves are still missing. (Ringo Starr's been on there for a while now.)
Eight Lennon albums have been posted on the iTunes Store, including remastered editions of Imagine, Mind Games, Rock 'n' Roll, Walls and Bridges, Lennon Legend, Plastic Ono Band, Sometime in New York City and Working Class Hero - The Definitive Lennon.
Many singles are available, and you can purchase the collections either using standard FairPlay DRM encryption or, for a modest additional fee, in iTunes Plus format -- a higher bit-rate AAC format that eschews DRM.