My thanks to Ken Rijock of World-Check for pointing out an upcoming change to US visitor registration processes. Starting from August this year on a voluntary basis, and mandatory from Jan 12th 2009, anyone who currently enters the US under the VWP will be expected to register online beforehand. This will replace the current I-94W form, but will have the added benefit that your electronic registration will be valid for multiple entries over a two-year period (or until your passport expires, whichever is sooner). The logic here is, then, slightly strange: currently, each I-94W you complete has to specify the address at which you will be staying while in the States, your airport of embarkation and your flight number. Those are pretty unlikely to remain the same for each visit over the two-year period... so for all but the first visit, that data will be obsolete and will contribute very little to the security of the homeland.
The announcement did not mention whether there will be related changes to the process for visitors leaving the US - a process which, over the last couple of years, has seemed to vary depending on your airport of departure (for instance, for a while, San Francisco airport had a rather half-hearted stab at collecting iris scans from departing visitors... but Chicago O'Hare and Washington Dulles didn't bother...).
Here's the relevant page at the Dept of Homeland Security.


