Point of fact, several suits by watchdog groups prompted this release, so this isn't exactly a "voluntary" move.
"As part of President Obama's commitment to government transparency, we are providing records of White House visitors on an ongoing basis online. In December 2009, we will begin posting all White House visitor records for the period from September 15th onwards under the terms of our new voluntary disclosure policy. In addition, as part of our new policy, we will post records dating from January 20th that are specifically requested on an ongoing basis. For more information, read the White House blog post announcing the new policy."
CREW, a liberal "watchdog" group filed suit in June of this year for the logs. So did Judicial Watch
A settlement was reached and this release is only a small piece of the pie. The rest is to come by year's end. So again, this is far from voluntary, and it's reproachable that the Obama administration would act as though it were otherwise.
Incidently, there are some interesting names such as Jeremiah Wright and William Ayers. Some are poo-pooing on the parade saying that they are just similiar names, but how many William A. Ayers are there anyway?