I read recently that John refused to play on George's songs after and including Revolver.
Does anyone know if this is fact and if so why ?
The most glaring example of John playing on a Harrison song post Revolver would be "For You Blue." Anyone who's seen "Let It Be" knows Lennon played the lap steel guitar with a shotgun shell (hence Harrison's quip of "Go Johnny, Go!" during the solo) on that song. He also sings backup on "I Want To Tell You," helped Harrison write "Taxman" and sang backup on said song, etc. Like a previous poster said, just do a Google search and you should be able to pull up the info.
Lennon never seemed to do much backup singing on George's songs, but I always felt it was because his register was too similar to George's; Paul's voice sort of naturally sounded better backing up George than John's did. But who really knows?
On a related note, has anyone heard the rumor about an alleged fist fight between John and George during the Let It Be sessions? I read somewhere that after George had his spat with Paul ("I'll play it how you want me to play it, or I won't play at all"), he got into a heated argument with John about John's increasing isolation from the group, which eventually came to blows. I've only seen this rumor in one place (Wikipedia), and haven't been able to verify it anywhere else.