The exemption threshold for the 2014 fiscal year for US residents on estate assets was $5.34m. Anything above is a flat rate of 40%.
http://en.wikipedia.org/wiki/Estate_..._United_States
Please note that this threshold and the tax rate are constantly changing & there is heaps of current debate. It is liable to keep changing.
My understanding is that the (1954) tax treaty between the USA and Australia means that we are allowed the same threshold as a US resident on any US based assets. I did read that document, but I can't remember where I found the bloody thing. There was a line in there that mentioned something along the lines of taxes levied on residents of both of those countries would be treated as if they were a resident of the country applying the tax.
There are also gift taxes. I don't think the US has exemptions for those to stop people from gifting assets before they die.
Also unsure if company / trust structures avoid the issue altogether.
I'm not 100% comfortable that this is the exact answer for individuals, and considering that the amount could potentially be big hit on the portfolio if I were to pass away after a very long and fruitful investing period I decided to limit my exposure to Australian domiciled ETFs (in particular VAS, VGS & VGE). It limits the options / flexibility for international exposure, but more peace of mind at this stage.