If anyone has run into this error in Xcode then I feel your pain.  The error message is not very helpful at all and it turns out that it occurs in many circumstances, none of which have anything to do with the Perforce root.  For me these were the symptoms - I'd just added a new, larger hard drive to my G5 and used the Migration Assistant to transfer the contents of my old hard drive to the new one.  After I did this, I started getting the following error in my Xcode projects that use Perforce:

 

No files in this project were found under the Perforce root:

 

These projects were working just fine before so they should be working now.  There's certainly nothing wrong with the Perforce root because my new hard drive contents are pretty much a clone of my old drive.  I've just been putting up with this for the last little while using P4V exclusively but finally I was fed up.  I started googling for solutions and the most common one seemed to be deleting your Perforce password in the Keychain Access application.  I tried this and it still didn't work, same error.  Then I tried running p4 info on the command-line and found that:

 

tcsh: /usr/local/bin/p4: Permission denied.

 

Doh!  That's a pretty stupid problem.  So I went into /usr/local/bin and sure enough, when I had reinstalled p4 I failed to set the permissions properly and the binary was not executable.  I corrected this by chmoding it to 755 and then fired up Xcode again hoping for it to work.  But no, I still got the same problem.  So this time I tried changing the P4CLIENT, user name and password settings of the project to garbage values. Then quit and reopen the project - same error.  Then I went into Keychain Access again and deleted the entries there (there was my original one and the new garbage one).  Again, quit and reopen the project and this time I was greeted with a Perforce error, the usual error that it gives you when you try to login with the wrong user name and password - this gave me some hope of success.  I went to the Perforce settings for the project and corrected the garbage that I had entered before and finally it started working.

 

I then tried openning some of my other projects that had the same problem and they all are working now (no Keychain funny business needed).  The moral of the story is that this is a pretty weird error message that can mean any number of things.  It could be that your p4 binary has messed up permissions or that the keychain entry for Xcode is messed up or a combination of these.  If anyone else runs into this problem then I hope this helps.