Brookmyre vs SF. Let's call it a draw
What makes it science fiction is not the type of story but the setting; to quote the good doctor again, that it "includes events played out against a social or physical background significantly different from our own". Anyone who reads SF knows that there's a lot of variety covered by this definition. Some books are really close to our own reality while some are vastly different. Some are more about the science than the fiction, others just the opposite.
Brookmyre says in his bio (presumably he writes it himself) that he "has established himself as one of Britain's leading crime novelists. This hasn't stopped people from nagging him to write SF instead." It's interesting, by the way, that he classifies himself as a crime novelist. Recent novels from "Chris" Brookmyre notwithstanding, I would have said that most of his output belongs in the "thriller" category, and that's exactly where Bedlam fits too. What we have here is not really a massive departure for him. It is easily identifiable as a Christopher Brookmyre novel in both style and substance. It's just set somewhere else that doesn't exist (yet) and so is SF too.
Unfortunately, as science fiction, it's fairly derivative. The notion of virtual worlds indistinguishable from the real world isn't new; the key texts here are William Gibson's astonishing Neuromancer (1982) and Neal Stephenson's wonderful Snow Crash (1992). The notion of multiple virtual worlds isn't original either: Ernest Cline's Ready Player One (2011) springs to mind (mainly because I'm reading it right now), but I'm sure there are others.
Also unfortunately, as a Brookmyre novel, it's average. This needs to be considered in context though - it's still good. The notion of all the virtual worlds being versions of old PC games is quite nice (and is very consistent with previous novels' obsession with first and second generation FPS games like Doom and Quake), the overall story hangs together and the characters are well drawn. It's as highly readable as ever.
I just feel that, in attempting to write SF, he's lost some of the essence of Brookmyre, but as SF it's not as detailed as I'd like. Still better than those dreary "Chris" Brookmyre crime novels though.
Update (2018): I've since re-read this book at least four times and thoroughly enjoyed it each time. I'm not quite sure why I was so underwhelmed the first time - maybe because the aforementioned Ready Player One was so good - but it's definitely worth noting that it's improved with age.