Just called BMWNA and the woman on the phone said she was unfamiliar with that provision.

It is not in the 2005 X's manual, so maybe it is only in pre-2005 vehicles. Does your manual have this statement?

Child-restraint system security
All of the rear belt retractors and the
front passenger's safety belt can be
locked for mounting and securing childrestraint
systems.
Information regarding this is located
near the buckle latch of each safety
belt.

If not, it probably points to a tensioner issue - although with the LATCH system I don't know why it would be an issue.

If you want to call BMWNA and ask them about your specific vehicle, here is the number I called and got right through on: 1-800-831-1117

They wouldn't give me any info from the 2004 manual b/c I have a 2005. Nazis.