The query below (with a generic Account Id) results in
2012-09-11 for a specific Task, but the date is displayed as
9/12/2012 when viewing the Task in the Salesforce interface. It seems all ActivityDate fields for all Tasks are different by a day.
SELECT Id, AccountId, Account.Name, Priority, Status, Subject, ActivityDate, Type, Owner.Name, Owner.Id, What.Name, WhatId, WhoId, Who.Name, LastModifiedDate FROM Task WHERE AccountId IN ('001000000000000')
The time-zone of the User accessing Salesforce through the interface and through SOQL is the same -
(GMT-05:00) Central Daylight Time (America/Chicago).