In 1.5, TabbedPanel#tabs has a wildcard signature: List<? extends ITab>. This makes it impossible to add tabs to that list, even though adding panels is one of the stated purposes of the #getTabs() method.
On #wicket, dashorst suggested giving TabbedPanel a type parameter <T extends ITab>, which I think is a good idea. It should definitely be added in 6.0.
What about 1.5.x? This would be a source-level break, but I think binaries should still work.