I was thinking in using some tools to convert the JAR into an APK. But will my Nexus One have enough memory? And what about the controls?
I know nothing about Minecraft but I can't see how it would be possible without rewriting big chunks of it. Even if you can convert the JAR to an .apk, there are many Java classes that are not part of the Android API. Furthermore, Android does not include AWT, Swing or Java 2D/3D, it has its own UI and graphics APIs, so the user interface would need to be rewritten.
Android cannot just "run" native Java applications. Android does not have a Java Virtual Machine. Android uses a Dalvik Virtual Machine, which is completely different.
While android applications are written in Java, it is a "subset" of actual Java. so even if you could run the jar files through a "converter", there is no guarantee the code is actually supported by Dalvik.