Many attendees from outside the US will require a Visitor Visa to enter the United States. See here for authoritative visa requirements from the US Department of State.
Attendees from the 35 countries in the Visa Waiver Program must still register with the Electronic Systems for Travel Authorization (ESTA) and possess an e-passport. Some with older pre-e-passports have found that it is easier to get a new passport than apply for a regular visa.
Most visa applications typically require an interview. The wait time to receive an interview appointment at a U.S. Embassy or Consulate could be over 16 days. Please see the US Department of State's page for estimates of visa and processing wait time at different consulates. Please schedule your appointment as early as possible.
If you require a letter of invitation, please indicate this when registering for ISWC 2012. The local organizer will send the letters either via email or post (as selected during registration) after the registration fees are paid in full. Please note that it will take mininum of 4-5 business days for an emailed letter and a minimum of 10-15 business days for a posted letter so please schedule your visa appointments accordingly.