On Aug. 1, 2008, the department will begin to accept voluntary applications through the ESTA Web site at https://esta.cbp.dhs.gov. Until ESTA is mandatory for all VWP travelers, however, ESTA applicants will also still need to complete an I-94W form en-route, for presentation at a U.S. port of entry.
ESTA applications may be submitted at any time prior to travel, and once approved, will be valid for up to two years or until the applicant’s passport expires, whichever comes first. Authorizations will also be valid for multiple entries into the U.S. To facilitate the authorization process,
このESTA(Electronic System for Travel Authorization)がアプルーブされると
最長二年は有効で(*パスポートの有効期限以内)複数回のアメリカ入国に使えるので、義務化される前ですが、早めにとっておくといいのではと思います。