With all other Mercurial repos that I push to, I can set my username and password in my hgrc (actually, Mercurial.ini since I'm using Windows) for doing operations at a particular domain, and Mercurial won't prompt me for auth info. However, even with these settings in place, Bitbucket always asks for my password.
My settings are as follows:
[auth]
bitbucket.prefix = bitbucket.org
bitbucket.username = myusername
bitbucket.password = mypassword
As I said, these settings work for all other repos I push to.