I hope he will. Only Forward is one of my all-time favorites. I tried scouring the web for interviews, but couldn't find anything conclusive.
Here's an old interview from 2001:
I'm not entirely comfortable with being 'science fiction,' which is how people often want to categorise me..."
which implies his shift away from sci-fi was intentional, although his official website is still www.michaelmarshallsmith.com. However, the "Smith" is greyed out, so he seems to be keeping the two personas distinctly separate, with the thriller-writer in the front.
In a newer interview, from 2010, he again refers to this split persona (with three identities, by now), but he in no way implies that he's done with the Michael Marshall Smith science-fiction writing hat.
My guess, then? When he feels like writing sci-fi again, he'll write sci-fi. No doors have been closed.