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.
Message: 2
Date: Thu, 28 Jan 2010 19:25:01 -0300
From: Victhor Foster <>
Subject: Re: [Gumstix-users] iTunes Remote Speakers
To: "General mailing list for gumstix users."
Message-ID: <>
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I didn't know there was such a thing, third party AirTunes products.