I will do a detailed explanation as soon as I can, but in the meanwhile, you can get a pdf of IBM's filing here. If you want the short version, read paragraphs 152 through 154 and the following relief section to the end.
What jumps off the page, aside from the copyright infringement counterclaim based on SCO's violation of the GPL, are paragraphs 33, 54, and 154.
In paragraph 33, IBM says:
"Indeed, SCO incorporated certain code licensed pursuant to the GPL into its proprietary Unix products."
Then in paragraph 154, they say they are entitled to the following relief, a declaration that:
". . . (3) any product into which SCO has incorporated code licensed pursuant to the GPL is subject to the GPL and SCO may not assert rights with respect to that code except as provided by the GPL."
Then in paragraph 54, IBM says SCO is asserting legal theories that are frivolous.
They covered all the bases. IBM has said to SCO all the things you wished someone would. I can't see one thing they could have used that they didn't use. It's a powerful document.
To be continued.