Check out Rogue Amoeba's Airfoil Speakers.  They have exactly what you're looking for, but I think they may only distribute their application pre-built for x86 linux.  Maybe you can cut a deal with them.
Bruce
Message: 2
Date: Thu, 28 Jan 2010 19:25:01 -0300
From: Victhor Foster <victhor.foster@gmail.com>
Subject: Re: [Gumstix-users] iTunes Remote Speakers
To: "General mailing list for gumstix users."
	<gumstix-users@lists.sourceforge.net>
Message-ID: <6F03FCA9-B961-497B-B7F5-F7E24FF3C477@gmail.com>
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I didn't know there was such a thing, third party AirTunes products.