The answer differs depending on what version of SharePoint you have. If you have MOSS, you can use "Target audiences" here (however, be warned that audience targeting just hides the item and users can still see it, if they use the direct url of an item).
Create an audience for each regional office and let the users assign items to audiences. To be sure that all items have the "target audiences" set, create an event handler that checks if the "taret audience" field is set in each item, if not - set it according to the current user.
If you have plain WSS or do not want just to hide but rather disallow access to items, you will have to BreakRoleInheritance
on all items and set permissions to all of them according to regional office the author comes from. You can do it in an eventhandler or in New/Edit form, your code options can be found here: http://stackoverflow.com/questions/2040699/sharepoint-capture-save-event-of-newform-aspx-edit-aspx/2040800#2040800