I may well open the source up once I'm sure it's working OK on most devices.. I'm not intending this to be a commercial app. On the other hand, I don't how easy it would be for some music players to just use the code - it depends a lot on whether they build their own media library or whether they use the Android default one.
In the future, I'd also hope to allow other apps to interact with this app via something called "intents" whcih is how Android apps talk to each other - so perhaps they could call my app behind the scenes, making it look seamless to the user.
Right now though, my priorities are, in order:
1. Make this work well on a wide selection of devices (you can help by letting me know what your device is, and whether it worked well for you - the areas that seem to vary a lot are: detecting the current track, displaying the track chooser, and sending the new tracklist back to the music player)
2. Improve the number of matches for each track
3. everything else...
"Everything else" vaguely includes: looking at making it work offline (which is really hard), interactivity with other apps, Genius mixes etc..