I was thinking of joining USAirways ff scheme. I originally hadn't bothered, because I always directed my US points to my Qantas account. And then I didn't bother because it looked like US would become part of UA anyhow.

Are there any sign-up bonuses, referral bonuses or anything like that?

If you are actually going to be flying US, check out the Official Bonus Code Thread. There are many ways to get multiple miles.

The only easy way I can think of to earn free Dividend Miles would be to sign up for an online DM Statement which would give you 500 miles.