Hmm I'm not so sure about that. 3.5mm is maybe the standard for computer-type speakers and earbuds... but banana plugs have been the standard for decades now in home theatre hi-fi systems. Pretty much any decent speakers will connect to a receiver using banana plugs. You also forgot XLR and 1/4" connectors which are the norm in production equipment (my cheapo M-Audio speakers are connected to my computer with those right now).

Whether the Nexus Q needs to contain a dedicated amplifier that can get around an A/V receiver (which a lot of people who invested in their home theatre system might already have) is another matter.