By law, the Social Security Trustees are the Treasury Secretary, Labor Secretary, HHS Secretary, the Social Security Commissioner, the Deputy Social Security Commissioner, and two public Trustees - outsiders.
Apparently Bush renominated the public Trustees last November to a second term, as their terms were expiring, but they haven't been reconfirmed yet. One of the excuses for not having a report being floated by Treasury is the fact that not all of the Trustees have been confirmed. It's difficult to see how lacking a full slate of Trustees would prevent them from issuing a report, especially as the "uncofirmed" ones just haven't yet been reconfirmed, but there you go...
...Yglesias notes that the delay is not without precedent, and if there are any political concerns they could also be due in part to a mismatch between administration/SSA immigration projections.
