In Howard Delivery Service v. Zurich American Insurance, No. 05-128, the Supreme Court ruled today that claims for unpaid workers comp. premiums were not entitled to priority status under then-Section 507(a)(4) (now subsection (a)(5)).
In a nutshell, the Justice Ginsberg wrote that workers compensation was more comparable to other types of business insurance (fire, theft, vehicle, etc.) than insurance provided to employees as a fringe benefit. Further, the insurance premiums were, in large part, for the protection of the employer as it protected it from large tort awards.
Read the opinion here (after a welcome screen).