If you run Sharepoint 2010 in one forest, users in another forest and a 1-way forest in-place. There is some additional configuration needed in Sharepoint 2010. I included links below that discuss the details. My post is not to be in-depth how to setup, rather share a tidbit not discussed in documentation (not that I could find). Thanks to a smart co-worker and process monitor, it was found there is a registry entry, the application pool needs READ access. You can either manually grant permissions on the server or add registry permission in AD Group Policy. Hope this helps.
People Picker overview (SharePoint Server 2010)
Configure People Picker (SharePoint Server 2010)
Peoplepicker-searchadforests: Stsadm property (Office SharePoint Server)
Application Pool needs read access
MACHINE\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\14.0\Secure
Multi Forest/Cross Forest People Picker
Microsoft MVP - IIS