I am using the stsadm property "peoplepicker-searchadcustomfilter" to set a custom search filter but the people picker returns no results, even though I am sure that the user exists. I am using a Custom Active Directory Attribute in my filter is there anything special I have to do to get it to work?
Yes, When the people picker performs a search it queries the Domain Controller using a LDAP Global Catalog Search Request. This means that when you create the new custom attributes you need to ensure that they are included in the Global Catalog, you can do this by checking the (2)"Replicate to the Global Catalog" option in the Attribute properties. If this option is not checked you will not be able to reference this attribute in your People Picker filter. Another tip which will speed up your queries is to make sure that you check the (1)"Index this attribute" option.
Remember that the custom filter will apply to PeoplePicker for the entire Web Application.