Setting Default DisplayCategory for Create Site Page

In MOSS 2007, one way to have your template category selected by default when click "Create Site" from Site Actions when Office SharePoint Server Publishing is enabled is edit 12\TEMPLATE\LAYOUTS\EditingMenu\SiteAction.xml (Backup first!). Edit the ConsoleNode item whose action is "cms:CreateNewSiteAction" and ID is "wsaCreateSite." Add (or replace) two attributes:
IsSiteRelative="true"
NavigateUrl="_layouts/newsbweb.aspx?DisplayCategory=MyCustomTemplates"

Note, the IsSiteRelative attribute is required so that subsites don't get file not found errors.

Finally, recycle the app pool.

Another option that applies to site collections only is to edit _catalogs\masterpage\Editing Menu\CustomSiteAction.xml, adding the Console tags with custom content as needed.