Use the following command to turn this configuration on;

Stsadm.exe -o setproperty –url http://server  –pn “peoplepicker-onlysearchwithinsitecollection” –pv yes

Use the following command to turn this configuration off:

Stsadm.exe -o setproperty –url http://server  –pn “peoplepicker-onlysearchwithinsitecollection” –pv no