CSS padding overrides overflow?

Whats happening here?

#agendaTitle{    margin:0;    padding:20em 0em 0em 0.75em;    height:3em;    overflow:hidden;    background-color:#ff00ff;}

The top padding is ridiculously high just to demonstrate - with a realistic requirement the div still increases height proportionally.

Surely the overflow:hidden means I should just see a block of colour?

Occurs in FF and IE.