If you ever need to migrate your IIS Web Site (Web App) to a new Application Pool in SPS 2003, here is a tip.... Ensure that you also migrate all the Sub-Virtual apps within the Web Site...

Failure to do so may result in « HTTP 403 – Access Denied » when browsing to your Portal. It may work if you browse directly to the page e.g. http://portal.com.au/default.aspx but may fail by simply going to http://portal.com.au