Enable that Cross-Domain People Picker, yo!

I'm writing this one definitely not as the first source on this subject, but mainly out of frustration.  Way too many Bing and Google searches looking for the right syntax, and way too many posts with the wrong string.

The following STSADM command will allow the People Picker to search for users in a situation where you have a one-way trust between domains.  This is especially useful if you're load testing your QA domain.

This command is all in one string.  Run this after you create a credential key.  Do that by entering this:
stsadm -o setapppassword -password <password> 

STSADM.exe -o setproperty -pn peoplepicker-searchadforests -pv "forest:QAFOREST.somecompany.com,AD\service-somesvcacct,thataccountpw; domain:ADFOREST.somecompany.com,AD\service-somesvcacct,thataccountpw" -url http://qa.somecompany.com

Explanations:

The service account used is the service account from the primary domain.  So in a one-way scenario, QA will trust it.

Once you run it, you can run the following to verify the setting was applied:

stsadm.exe -o getproperty -pn peoplepicker-searchadforests -url http://qa.somecompany.com

If you need to clear the property, you can run this:

 
stsadm.exe -o setproperty -pn peoplepicker-searchadforests -pv "" -url http://qa.somecompany.com  

Hope this helps someone.