In the menu bar, there's a simple Siri black and white icon that features the word "Siri" surrounded by a box, while the full dock icon is more colorful and features a colorful Siri waveform in the style of other built-in app icons.
I’m definitely not into the dock icon for Siri, if that is what it ends up looking like. It just doesn’t seem like it’ll fit in alongside the other icons in the dock at all.
Also, as John Gruber pointed out on Daring Fireball, why would Siri on Mac have not only an icon in the dock but one in the menu bar as well? It doesn’t make a ton of sense to me but, with the feature obviously still in the early stages, maybe final placement hasn’t been nailed down.