The iPhone/Android app called "Apptui" recently added strong support for Magnatune.
I've been using (and testing) it these past few weeks, as their Magnatune support went into beta, and I'm pleased to say I'm finding it quite useful. It's free, and downloadable here.
What does apptui do?
Here is a screen showing the basic idea:
Apptui turns your smartphone into a remote trackpad to your web browser (that's a generic feature for every web site) and then it also adds site-specific navigation and control buttons for Magnatune.
Here's a typical use case.
Let's say you are playing music at Magnatune.com, in your apartment, and step away to make dinner. With your smartphone, you can easily use the stop/start/prev/next buttons to control the music playing. If you don't like a track, skip it. Or use the stop/start button if you want some quiet.
There's also a built in search button, and menu buttons to jump among the genres:
anything you can't do with the magnatune-specific components, you do with the screen of your smartphone, which works like a remote trackpad.
All in all, it's a bit like Apple "remote" app for iTunes. It's not a general replacement for the Magnatune iphone/android app: it fills a different need, of remote controlling your browser's session with Magnatune. And FYI, it works both on the free and member sections of magnatune (i.e., http://download.magnatune.com)