"I'm testing on Ubuntu 12.04 with X.Org X Server 1.11.3.
I tried Johan's solution and it appeared at first to work, but I found on my system the text after ""local:"" was being ignored. The result was that all local users were given access to my X session rather than one user.
The magic I found worked was:
Replace the portion after the last colon with the username you wish to grant access. You can inspect the authorised list by running xhost with no paramters. It should look something like:
access control enabled, only authorized clients can connect SI:localuser:masi SI:localuser:myself
This indicates that the ""masi"" user and the ""myself"" user are being granted xhost permissions in the same way."