Why does the Century font in Pages '09 not include italics or bold?
It probably means you don't have the bold and italic versions installed. You can get those versions by downloading them (here, for example). Just Google search for century bold and century italic. 

Fonts are all basically just collections of images. Bold and italic ones are actually completely different fonts, just sometimes they're packaged together rather than as separate fonts. Some fonts don't have bold or italic versions at all. That's why some fonts don't have those options sometimes.