Sony PS2 - trademark infringement

IBM copyright and trademark information: "PS/2�"
So if PS/2 is a registered trademark of IBM, why not sue Sony?
Yeah, I'm bored.
My aunt has an IBM Personal System 2 (from long, long ago); I beleive this is where they came up with "PS/2" from. Funny that they don't assert trademark rights over "Personal System 2"

PS: Well I guess the "/" might make a difference, but I don't know.