It is very easy to add a breacrumb to your custom application page.
First of all you need to create an xml file in the
C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\ folder.
Rename this new xml file like this
layouts.sitemap.*.xml where you can change * to any string you want.
then change the content of the XML file like this
STSADM -o copyappbincontent