Sports Illustrated is reporting it. No players mentioned at this time, but I suspect the A's no longer have a glut of Travis Buck-esque outfielders, and 2B Eric Patterson needn't worry about Mark Ellis' contract extension.
[UPDATE] From Ken Rosenthal:
Over the weekend, the teams discussed A's outfielder Carlos Gonzalez, left-hander Greg Smith and reliever Huston Street, according to one source, but it is not known if any or all of those players will be included in the package for Holliday.