You will have to update the address each time you visit unless you are staying at the same place apparently.
the first time you do it you will receive an ESTA number which the airlines will require before you fly and this is the numer (once you have received approval) which SHOULD provided everything is straightforward be issued vitually instantly once they have reviewed your infor. Then when travelling you will not need to fill in the green form on the plane once it is full yoperational.
you can reigster voluntarily from 1st Aug this year but from (12th?) January 2009 it will be compulsory , at the moment registration is free but they are expecting to bring in a charge ($10 has been mentioned) at a later date.