This is applied to Sitecore 5.3 061102. If you create a folder (template container) for your custom templates in the Template Manager, you might not see this folder in the dialog when adding an item from a template or changing the template.

The solution is to edit the \sitecore\shell\Controls\Data\TemplateDataContext.xml file to include the GUID of the template "Template Folder" that is placed under /sitecore/templates/system/templates/.

The Filter attribute of the DataContext definition should be edited:

Filter="Contains('{E3E2D58C-DF95-4230-ADC9-279924CECE84}, {...}, {...}', @@templateid)"

Filter="Contains('{0437FEE2-44C9-46A6-ABE9-28858D9FEE8C},{...}, {...}', @@templateid)"

where {0437FEE2-44C9-46A6-ABE9-28858D9FEE8C} is the GUID of the "Template Folder" template.