I believe the major problem with pair programming is that it makes the client think that 2 people are working one a task that should be done 1 person instead. What can I say to the client to convince them peer programming actually boosts productivity?
Seriously, I think that if you can get some stats regarding reduced faults in pair programmed code, that may work. I think that you could also mention some of the other benefits of XP such as collective code ownership, clients will be less concerned about staff turnover and holiday absences, etc.
I guess that I'm advocating pushing XP as a package rather than just one part if it.