I'd favor putting things in SO or SF if they belong there, so they won't get lost in the SU dumping ground. This means I'd generally be happy with desktop admin questions in SF, as long as they aren't things that would be unlikely to be in the corporate environment.
The ServerFault FAQ says that the site is intended for people who administer a large number of desktops that aren't their own (among others), and that questions referring to those desktops are welcome.
Obviously, there's going to be some overlap of problems between somebody who administers his or her own private computer (and belongs on SuperUser), and somebody who administers a lot of corporate systems (and belongs on ServerFault). I'd say a question should only be moved to SuperUser if it's a question that most corporate IT admins would not find useful.
Edit: Here is the beginning of the ServerFault FAQ. I certainly hope it's accurate; I copy-pasted it. It is my understanding that this is supposed to be the official position.
Server Fault is for system administrators and IT professionals, people who manage or maintain computers in a professional capacity. If you are in charge of ...
- many desktop PCs (other than your own)
... then you're in the right place to ask your question! Well, as long as the question is about your servers, your networks, or desktops you support, anyway.
Please note that Server Fault is not for general computer troubleshooting questions; if you paid for that desktop hardware, and it's your personal workstation, it is unlikely that your question is appropriate for Server Fault.
This appears to mean that questions about supporting corporate desktop PCs are on topic for people who have that as their job responsibility. It says that if the problem is on your personal machine, "it is unlikely that your question is appropriate for Server Fault." I am skilled and experienced in the English language, and the words in the FAQ are not unusual and do not appear to be arranged in a confusing manner. My conclusion is that they likely mean the obvious meaning.
I would be very interested in learning which questions pertaining to administering corporate desktops are not appropriate for SF, and why labeling such questions off-topic is not against the FAQ.