When you publish an iCal calendar, the events you've created appear using the time zone set in Date & Time preferences. If you change time zones, follow these steps to make sure the published calendar reflects the new time zone:
Open iCal.
Export the calendar to the desktop by choosing Export from the File menu.
Unpublish and delete the calendar in iCal. (Don't worry. It's saved on the desktop.)
Double-click the icon of the calendar you saved on the desktop and add it back as a new calendar.