This post is a good reference for the abilities of some audio dock connectors :
http://blog.faberacoustical.com/2009/iphone/dock-connector-audio-io/It seems that if you plug your headphones after the dock connector, headphone output will be selected by the OS :
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Connecting headphones, or an adapter cable, like a stereo mini-plug to RCA adapter, will cause the headphone output to be selected
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Using a dock connector input with the original (1st generation) iPod touch appears to require that something be plugged into the headphone jack, unless the dock connector device also supports audio output
But I don't have any of these yet, so I can't confirm.