It appears that the Amazon MP3 app extracted from the latest Nexus One (v1.6) will install quite successfully on a rooted Liquid (probably other handsets too) and provide full UK Amazon MP3 support for those of us in Blighty. Just push it into /system/app using adb and then (if you're in the UK) change the locale to en_GB using More Locale since for some reason the Liquid only allows you to set en_US locale for English users from the Settings app. Poking around inside the APK there also appear to be resources for French and German locales so it may provide full local support in France and Germany too. A Google for nexus com.amazon.mp3.zip will show you where to obtain it on xda-developers.
Edited by Uxian, 16 January 2010 - 12:54 AM.