Quoting from the IBM official announcement:
IBM today announced the company’s mainframe and POWER-based virtualization technologies have achieved one of the computer industry’s most stringent security certifications, illustrating IBM’s virtualization leadership.
In the security certification — known as the Common Criteria’s Evaluation Assurance Level (EAL) — IBM achieved one of the highest levels of certification for logical partitioning, IBM’s premier virtualization technology used by its servers to run multiple operating systems simultaneously.
IBM’s mainframe logical partitioning technology received a Level 5 rating, IBM’s POWER-based virtualization received a 4+ rating.
Before IBM only VMware submitted its products to Common Criteria evaluation, obtaining the EAL 2 certification for ESX Server 2.5.0 and Virtual Center 1.2.0.
The value of Common Criteria is concrete, even if a great security expert like Bruce Schneier have a different opinion.
As Richard Bejtlich correctly noted everything depends on Protection Profile, which can be declared in a sharp way to easily obtain the certification.