UPDATE – more details of the brief have come out –

In essence, the position of the federal
government would simultaneously give some support to marriage equality
while showing some respect for the rights of states to regulate that
institution.  What the brief endorsed is what has been called the
“eight-state solution” — that is, if a state already recognizes for
same-sex couples all the privileges and benefits that married couples
have (as in the eight states that do so through “civil unions”) those
states must go the final step and allow those couples to get married. 
The argument is that it violates the Constitution’s guarantee of legal
equality when both same-sex and opposite-sex couples are entitled to the
same marital benefits, but only the opposite-sex couples can get
married.

The eight states that apparently would be
covered by such a decision are: California (whose Proposition 8, which
denies marriage to couples who already have all of the other marital
benefits, would fall), Delaware, Hawaii, Illinois, Nevada, New Jersey,
Oregon, and Rhode Island!

PLUS Attorney General Eric Holder released a statement about the filing:

“In our filing today in Hollingsworth v.
Perry, the government seeks to vindicate the defining constitutional
ideal of equal treatment under the law.  Throughout history, we have
seen the unjust consequences of decisions and policies rooted in
discrimination. The issues before the Supreme Court in this case and the
Defense of Marriage Act case are not just important to the tens of
thousands Americans who are being denied equal benefits and rights under
our laws, but to our Nation as a whole.”