I don't think that xDB can work in a J2ME environment out of the box. The main problem with J2ME is that it is a very limited environment: it lacks many classes that are available in J2SE and it also supports only 1.3 (in some environments 1.2) class format. The latter means that >= 1.4 Java features (assertions, generics, static imports etc.) are not supported.
What this means is that to use xDB with J2ME you would have to backport xDB (which is Java 6-based) to Java 1.3/1.2 using a tool such as http://retrotranslator.sourceforge.net/ and then provide backported versions of the J2SE classes that are not present in J2ME. But even then I cannot guarantee that xDB will work (probably not) - we have never tried it in the past.