My two cents…
Tell them you secured the domain with the intention of marketing Gold Nugget Casino and did not forsee a conflict. Since you have already invested time and development monies you cannot relinquish ownership without reasonable compensation. I would ask them for an amount between $5k – $10K to transfer the domain to them and avoid costly litigation fees for BOTH sides.
If you present it like that they may agree to a Nuisance settlement to avoid court costs. If they dont and want to play rough you could be in for a rough ride that would require you to retain an attorney. If it comes to that let us know as several members work with US based attorneys who specialize in internet law and Intellectual property.
The Mirage people have billions of dollars in assets and an army of attorneys, being in Australia will not help or protect you, so ignoring them is probably the WORST thing you could do.
Good Luck