The Financial Accounting Foundation (FAF) has announced that the SEC has adopted the 2011 U.S. GAAP Financial Reporting Taxonomy (UGT). The 2011 UGT contains updates for accounting standards and other improvements to the official taxonomy previously in use by SEC issuers.
The taxonomy is now listed on the SEC's Standard Taxonomies page, which includes this statement on adoption:

The SEC staff strongly encourages companies to use the most recent version of the US GAAP taxonomy release for their Interactive Data submissions to take advantage of the most up to date tags related to new accounting standards and other improvements. However, due to the timing of the release of the US GAAP 2011 taxonomy in conjunction with the phase-in of the Rule, companies will be permitted to continue to use the US GAAP 2009 taxonomy.

In 2010, maintenance for the US GAAP taxonomy was transferred from XBRL US, the American jurisdiction of XBRL International, to the FAF. The new taxonomy was originally proposed on September 1, 2010, and the period for public review and feedback ended on October 30.
The new tags update both the Commercial and Industrial Taxonomy, applicable for most companies, as well as industry-specific taxonomies. Major changes to the taxonomy are summarized in the Release Notes beginning on page 2. Altogether, approximately 2,000 new elements have been added and several hundred have been deprecated, i.e., elements which “…remain in the taxonomy to satisfy legacy and conversion requirements but should not be used in filings based on the 2011 UGT.”
XBRL US provides links for viewing and downloading the taxonomy on its 2011 US GAAP Financial Reporting Taxonomy page. To view the taxonomy, use the links under XBRL US GAAP Taxonomies Viewer at the right of the page. The UGT can be downloaded – including an Excel version – using links under the Essentials heading at the bottom.