Well I didn't see this feud coming:
According to various websites: Apparently Billy Joel made some snide off-hand comment to the MSG staff about Phish having booked MSG for new years. I guess the story goes that he tried to book it in October and was told that Phish had it on lock. His response was something to the effect of them being a "no talent cover band".
Phish who is are known for never backing down for a challenged played all four nights at MSG with original songs. No covers. No Repeats.
Your turn Mr. Joel. Play four straight shows and don't repeat one of your "hits".
Phish - 1
Joel - 0