It's the antithesis of Wally. Wally defies the system by carrying a mug around letting everyone know he doesn't do any work, while the rest of us have to carry around a piece of paper to keep up the appearance of doing something.
The 'joke' is as plain as the paper on Slug's face. I work with several colleagues that today's 'Dilbert' is alluding to but I've got to say - the slug's better looking and has more charm!