Most airlines have a policy regarding passengers with medical conditions. Whilst it relies on a degree of self-disclosure, in your case, it's going to be pretty obvious. Airlines have the right to set limits on carriage (for example, most international carriers refuse to take women who admit to being more than 36 weeks pregnant). In most cases, airlines will permit passengers to fly with a letter from their doctor. Note that some airlines insist on the doctor completing one of the airline's own forms (which they usually fax to the relevant doctor). From the airline's perspective, declining to transport a passenger with a medical problem is safer, cheaper and far more convenient, than a diversion and unscheduled landing.
You should consult your doctor about possible complications whilst flying, and in particular, whether or not there is any chance of upper limb and wrist swelling occuring. You should also consult your doctor to determine what signs there are of an over-tight cast, and what steps should be taken in the event of these signs occurring. Other issues you should clarify with your doctor include a letter indicating the nature of injury you have (including date of injury, site of fracture, initial management, initial Xray findings, type of cast), and a letter to cover you for any analgaesic (if you are on any), as both US and Australian customs deem prescription analgaesics to be 'controlled substances'.
In terms of your insurance, you need to determine whether or not you are covered (in the event of a pre-existing policy), or will be covered (in the event where you are not covered by an existing policy). Remember that even a simple trip to a public hospital Emergency Department in the US may set you back literally hundreds of dollars.