Apparently he cuts shot of the monolith to the eye of HAL. Meaning the monolith really have an effect on the computer's "mind", like it did with the apes (read that on one comment)
It's funny because I always thought it was super obvious that it was a big part of the story (even the shape around the eye of HAL looks like the monolith, and I'm sure Soderberg did not miss the opportunity to play with it)... until I read the novel last month and find out that, no, the HAL going nuts segment have nothing to do with the monolith. It really is just a problem of HAL having trouble lying to humans because the mission needed it. What a letdown, lol. (but the movie do not emphasis this side of the story, so it really leaves the audience to think what they want)
(Side note: I'm reading 2010 right now and it's funny because C.Clarck wrote in fact a sequel to the movie. Not one of his own book! Several scenes from the Kubrick's movie that are not in his book are mentioned, as well as some change of settings (the huge monolith floating on space, instead of being at the surface of a planet, etc..)
I read that in the other two books, 2061 and 3001, he also made changes to the story to be more scientifically accurate.)