GPO provides the capability to download XML in bulk for select collections from the [FDsys Bulk Data Repository] (http://www.gpo.gov/fdsys/bulkdata/).
Please see the User Guides within this repository for more information on how to make use of the XML data for the available collections.
Examples:
Federal Register Issues from September 2012: http://www.gpo.gov/fdsys/bulkdata/FR/2012/09
Code of Federal Regulations: http://www.gpo.gov/fdsys/bulkdata/CFR/
Bills back to the 113th Congress: http://www.gpo.gov/fdsys/bulkdata/BILLS