Two options to editing the Portal Welcome Portlet:
1. Take the welcome portlet par and change the text
2. Pg 35 of: http://support.sas.com/rnd/itech/updates/91/portal_dev.pdf
Never tried this myself though … I usually just log in as sasguest and edit the entire Public Kiosk page. (Removing the Welcome Portlet and adding a Message Portlet that points to a code snippet or url with text.)