dtb, sorry for the - hopefully not too bad - joke with the "programmers language". I just couldn't resist. Here is info about it directly from the IRS:

irs.gov - Some nonresidents with US assets must file estate tax returns

irs.gov - Estate tax for nonresidents not citizens of the united states

As the first link says: Estate tax treaties between the U.S. and other countries often provide more favorable tax treatment to nonresidents by limiting the type of asset considered situated in the U.S. and subject to U.S. estate taxation.

But that of course depends on the individual's country of residence. New Zealand for example has no inheritance tax and because of that no double taxation treaty with the US covering such. Therefore if one partner of a New Zealand couple who has all of their assets in BRK dies their whole assets, all of their BRK stock, is subject to the full 40% US inheritance tax instead of the New Zealand 0% and the surviving partner gets just 60% of what they owned together.

There are ways around that. If you know you'll die as Blackswanny says liquidate everything before and transfer it - in this example - to New Zealand.

Another way is this tip which a fellow board member once gave me: As this law applies to legal persons only one can get around it by setting up a structure where not a person but a different entity, a trust or so, owns the US assets, the BRK shares or whatever. When the person dies it does not constitute an inheritance case because the owner is the trust, and that trust did not die.