I definitely think steampunk has a place under sci-fi but wanted to ask before starting any possible discussions.
Steampunk usually involves some kind of alternate history of technological development so it is feels closer to science fiction than it does it fantasy. I don't see a good reason to exclude it.
Steampunk, like the other "punks" is one of those hate-it-or-leave it genres. I like it, and I consider it "on topic" as it tends towards "alternate histories."
I'm not generally fond of steampunk myself, but I would agree that it's on-topic.