Originally Posted by recluse
Interesting. http://www.wowwiki.com/API_FocusUnit - I had no idea that they removed such a command. From the sound of it, you can't even call it from a secure action button object, which is silly. Evil bots ruining it for us. 
As an alternative, you could just put /focus into a macro and drag it to an action button. |
http://www.wowwiki.com/SecureActionButtonTemplate
Focus can be set as a type attribute for SecureActionButtons. Gladius/Proximo have used this for right-click focusing on frames.