What is required for an involuntary admission to a psychiatric
hospital in the Netherlands?  My wife suffers from bipolar disorder,
is currently having a severe manic episode, refuses to take her
medicine, and is quickly becoming a danger to herself.  We are on
vacation in Amsterdam and I would like to be prepared should a
hospitalization become necessary.

I am Dutch myself and I will first answer this as though you could
read the info in Dutch, and after that, translate some essentials.
First of all the law (Special Admissions Psychiatric Hospitals):
http://web.inter.nl.net/hcc/F.Strijthagen/wrdb/bopz.html
And an explanation of this law:
http://www.hulpgids.nl/wetten/bopz.htm
Necessary conditions for involuntary admission to psychiatric hospitals:
* the person has a "psychiatric condition"
* this condition causes danger to the person him/herself, others, or
the general safety of people and goods.
* the problem cannot be solved by any other person/institution.
* the person does not show a willingness to get treatment.
Most likely the procedure goes via a judge, but before him the DA, so
in short, via the police.
I would not hesitate to explain your situation to the police *before*
it becomes an emergency, and I am sure they have handled such
situations before.

Good luck.