We've had some issues getting the people picker to behave the way want recently. The Active Directory environment where I'm working has a number of forests in those forests the users have a rich AD schema. We wanted to filter the people picker based on the properties in this schema, the first thing we did was to construct an LDAP query that satisfied our needs. Then we used the service pack 1 feature to enable this query to be run, taken from Joel's Blog:
peoplepicker-searchadcustomquery
We found that this wasn't working, we knew the LDAP query was working fine, we use this same query in the profile import process.
After a bit of hunting around we found that our custom AD schema changes weren't being replicated and indexed by the global catalog, we found this Microsoft resource that provides the following sequence diagram on how the people picker dialog works:
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Notice how the first thing the WFE does after the user clicks search, it does a Global Catalog search. So by promoting the schema attributes to the global catalog we can get our LDAP query working properly. Now this probably won't apply to most people, but it sure is handy to know.