The first public alpha of the Mobipocket Reader for Java Phones is available here
This version has been developed for all Sony Ericsson Java Phones, but at least DRM-free e-books apparently also work on Nokia phones.
The Mobipocket Reader 6.0 Alpha requires a phone with J2ME and the JSR 75.
This Alpha version of Mobipocket eBook Reader is not supported and stable yet. Do not use it in a production environment : it may crash your device, so please make sure that you have a backup of your data before installing the Reader.