General Visa Information
The site http://travel.state.gov/ has information about obtaining a visa for those traveling to the United States. A similar site is http://www.UnitedStatesVisas.gov/ Both have links to the individual websites for U.S. embassies and consulates worldwide. The embassy and consulate websites have very helpful information about procedures, timelines, communities served, required documentation, and fees.
Letters from ACM
Letters are issued from ACM headquarters for attendees to
submit in support of their visa applications. For those who
are authors of papers, speakers, tutorial leaders, or have
other specific responsibilities in relation to the
conference, this information can be included in the visa
application letter. Please send your request for a visa
application letter to Ginger Ignatoff (firstname.lastname@example.org),
and include your name, mailing address, and fax number, as
well as the name of the conference you are attending.