As far as I can tell, the steampunk genre is defined by steam technology and brass, and a rather high Victorian camp sensibility. Recall Jules Verne and H.G.Wells's original sci-fi stories, and you'll know exactly what that means. Of course, over the years, aficionados and practitioners of the genre have developed it, and that means that there will always be the odd one who, by widespread acclamation, would have 'extended the boundaries'.
And so, by adding a bit of tongue-in-cheek, or self-reference, or by adding social commentary, or by ignoring, say, the brass aspect, a writer would have pushed the said frontier.
Tongue-in-cheek and social commentary combine in James Morrow's 'Lady Witherspoon's Solution'. Sexual slavery - both human and machine - is explored in 'Machine Maid' by the Australian writer Margo Lanagan, which also functions as a tale of crime and mystery. Genre-bending, anyone?
The other stories are a bit more uneven. Ian MacLeod is a supreme wordsmith (his The Light Ages was a literary marvel, the steampunk aspect being the least of its wonders), and in 'Elementals' he melds the fantastic with Victorian science. But tales such as 'The Dream of Reason' by Jeffrey Ford are a bit weak - it has little to do with steampunk other than being based in a vaguely Victorian setting.
Unevenness in anthologies is expected, I guess, and because there are good stories in the blend, I can safely say this is an OK book.