decompiling DEX into Java sourcecode

Question

How can one decompile Android DEX (VM bytecode) files into corresponding Java sourcecode?

Answer 1

A lot has changed since most of these answers were posted. Now-a-days there a are many easy tools with GUI's, like these:

APK Easy Tool for Windows (GUI tool, friendly)
Bytecode Viewer - APK/Java Reverse Engineering Suite
URET Android Reverser Toolkit

Best place to find them is on the XDA Developers forum.