The organizers of ISIT 2004 advise all travelers from outside the US to make sure they either qualify to enter the US under the Visa Waiver Program or to apply for a visa, as soon as possible. Information about visa and passport requirements is posted at http://www.travel.state.gov/ On the website, see Visa Waiver Program for the list of 27 countries participating in the visa waiver program. Note that travelers from five of the countries, including Belgium and Slovenia, will need a machine readable passport in order to enter the US under the program. (This requirement will become effective for all 27 countries on October 27, 2004.)
Also on the website see Visitor Visas and the Notice on Current Visa Processing Situation. It states that personal appearance interviews may be needed in some cases, and recommends "that individuals build in ample time before their planned travel date when seeking to obtain a visa."