Try the following (assuming that the component Id of your tabbed panel is "tabbedPanel1").
<xp:button value="Create New #{javascript:var tabbedPanel = getComponent('tabbedPanel1'); var selectedTab = tabbedPanel.findComponent(tabbedPanel.getSelectedTab()); return selectedTab.getLabel();}" />
So if you had tabs labeled "Contact", "Company", "Task", and "Message", your button's label would be "Create New Contact", "Create New Company", etc., based on which tab is selected.