Last year, the first report I can find
was on January 19th (from cornell). The first big wave seems to be in early February (see the link to get an idea of the timing last year -- pay attention to the year, since it jumps from April 2007 to January 2008 on that page). Also check last year's profile thread
. Many of us updated our profiles with the dates of admissions results.
EDIT: Also see the 2008 acceptances thread
. There are also slightly earlier results than the 19th reported on the profiles page. Also take a gander at this thread
about the opposite side of the coin...