If you try to type Brazilian Portuguese, you might be puzzled by the OS X keyboard layout that comes with the Brazilian flag attached to it. It's neither Portuguese nor Brazilian, but simply the normal US layout, where you type accented characters using Option deadkeys, according to this chart.
As I understand it, Brazilians are used to typing with a layout called ABNT or with the Windows layout called US International. You can get an ABNT layout (winbrazabnt.keylayout) from my iDisk, and the US International layout can be found here. Note that OS X 10.6 comes with US International already installed.
This graphic shows the 5 layouts provided with Mac and Windows for European and Brazilian Portuguese.