Don’t know the full story yet, but on multiple Facebook pages our team has visited today we’ve noticed that you cannot comment on the wall postings.
An example from Social Media Examiner:
Versus the recent comments:
For reference here’s more of the page in context:
Update: This does not affect walls of personal profiles. Just Pages, no word yet if this affects community pages yet.
Update 10:09 am Target’s Wall: http://www.facebook.com/target?v=wall documents this quite well
Update 10pm: Known issue, Social Media Examiner has a thread explaining that its the posts made by fans to the walls of business pages. However fans can still comment on posts by the page admins. AllFacebook.com is also covering it.