Recently, I had to extend the PeoplePicker’s functionality to include lookup’s in a one-way trusted domain.  I found some great posts but decided to consolidate into one post as there were a couple of caveat’s (a.k.a. ‘Command Line Error’s in stsadm).

1. Open a command prompt and change your directory to C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN.
2. Run the following STSADM command:  stsadm.exe -o setapppassword -password [insert a random password].
   • The random password can be anything you choose. (It’s used to encrypt/decrypt the password connecting to the other domain/forest)
   • The above command must be run on each web front end of your farm installation.
3. Run the following STSADM command:
   • stsadm.exe –o setproperty –pn peoplepicker-searchadforests –pv <list of forests or domains> -url <webapp>
     • <list of forests or domains> could equal Domain:MyDomain.com or Forest:MyDomain
     • example:  stsadm.exe –o setproperty –url http://testapp –pn peoplepicker-searchadforests –pv “domain:testpoc.com,username,password”
4. You can add multiple domains/forests by separating everything within the –pv value with a semi-colon….which is probably a good idea if you are adding in the domain/forest connected via the one-way trust.

Those are the two commands that you need to run in order to lookup users in another domain that is connected by a one-way trust.  For more information, please refer to the following link:  http://technet.microsoft.com/en-us/library/cc263460.aspx