I have an EeePC with Moblin 2.1 (release candidate) installed on it (via the packages in http://repo.moblin.org/moblin/releases/2.1/ia32/os/). By default, you don't get an mp3 decoder with this distribution. It wasn't obvious (when I googled for it) how to install the right GStreamer plugins: several blog entries suggested a variety of pretty hacky methods and work-arounds.
Then I chanced on a blog entry which mentioned that Fluendo have a free legally-downloadable RPM of their own mp3 encoder; I remember someone at work mentioning it too. I downloaded and installed this onto my netbook with no difficulties. You have to register for an account and go through a faux shopping basket to get it, but it is fairly painless. Here's the product page URL: