I am currently working with Siebel Call Center (Siebel Tools 8.1) and I'm having some trouble adding a tab to the top of the screen, where the 'Accounts', 'Contacts', etc tabs are.
I have added an item under the Page Tabs menu for the Siebel Universal Agent application, but the tab will only show up for the Siebel Administrator.
This is likely because you have not associated any responsibilities with the tab.
This can be administered under:
Site Map > Administration - Application > Responsiblities > Tab Layout
More detail can be found here:
http://download.oracle.com/docs/cd/B40099_02/books/Secur/Secur_AccessControl40.html