Anyseen this on Skysportsnews, shows it kicking off big time on the pitch, you can only see it from far away though, but these are the only pictures iv'e found:

http://cache1.asset-cache.net/xc/80868466.jpg?v=1&c=NewsMaker&k=2&d=17A4AD9FDB9CF193CE41B024AE96D64DEEAA58B2F6D6542E 284831B75F48EF45

http://cache2.asset-cache.net/xc/80868056.jpg?v=1&c=NewsMaker&k=2&d=17A4AD9FDB9CF193CE41B024AE96D64DCA8D7181D4809938 E30A760B0D811297

http://cache3.asset-cache.net/xc/80868007.jpg?v=1&c=NewsMaker&k=2&d=17A4AD9FDB9CF193CE41B024AE96D64D09D83B4A57D9589F E30A760B0D811297

Apparently the United players were warming down and the Chelsea groundstaff asked/told them to leave at which point a scuffle broke out.