For those who are interested in Steampunk writing specifically, the best forums I've found are:

http://brassgoggles.co.uk/forum/

http://www.thesteampunkempire.com/forum

http://steampunkwriters.ning.com/forum

The first two only have one of several forums dedicated to writing, and the last is entirely dedicated to Steampunk writing, but doesn't have the user base of the others.