Nitzan Raz has a great article on some lesser known people picker settings over at http://blog.oneboredadmin.com/2012/09/some-things-i-didnt-know-about-people.html

There are two people picker tweaks I am aware of – one of which is set on the site collection object (with set-spsite -UserAccountDirectoryPath, which seems to limit the people picker to a single OU in AD, but only for the given site collection)

The other tweaks are discussed in Nitzan’s article, and involve the web application level (for those who can’t keep the terminology straight, this is the highest level, I often think of this as the IIS site.

Grabbing the SP Site object with get-SPWebApplication lets us look at the PeoplePickerSettings

$wa= get-spwebapplication http://yoururl.yourdomain.com
$wa.PeoplePickerSettings

Update July 11 2013:
I had an issue today where users in a trusted domain weren’t showing up in the people picker in a stage environment, but they worked fine in production.
The difference was $webapplication.PeoplePickersettings.ActiveDirectoryCustomFilter which was empty in the working environment, but had a value in the non-working environment.

The fix was like this:

# remove the filter
$wa = get-SPWebApplication http://www.peoplepickerdoesntwork.com
$backup = $wa.PeoplePickersettings.ActiveDirectoryCustomFilter
$wa.PeoplePickersettings.ActiveDirectoryCustomFilter = $null
$wa.update()
$backup | out-file originalpeoplepickercustomfiltervalue.txt