The Office of the Solicitor General has filed its brief on the severability issue, which you can access here.
As it has in the lower courts, the United States argues that, if the Court declares the minimum coverage provision unconstitutional, the rest of the ACA should remain in force except only for the guaranteed-issue and community-rating provisions that go into effect in 2014. The United States also argues that the challengers lack standing to seek the invalidation of most provisions of the ACA, thus rendering most questions concerning the continuing validity of the ACA (were the individual mandate unconstitutional) non-justiciable.
UPDATE: It is not just constitutional standing principles that the United States believes prevent the Court from considering the invalidity of most other parts of the ACA. Rather, it is virtually every constitutional or prudential doctrine related to the breadth of judicial review. Check this out (from pp. 15-16): "Whether viewed as a matter of Article III and prudential standing, a limitation on the scope of equitable relief, application of the principle that facial challenges are disfavored, or simply a matter of judicial restraint, the Court should not consider the validity of provisions that are not directed to petitioners but instead affect numerous parties not before the Court."
