If you are a J2ME programmer writing commercial Midlet applications, then you must seriously consider signing your Midlet JAR Files.

Signing your JAR files helps the client know who has written the application as the midlet is considered to have originated from a trusted source. Signing also helps you overcome many restricted functions.

I came across a useful tutorial on how to sign your Midlet JAR files.

I recommend you to read this article: http://www.spindriftpages.net/blog/dave/2006/06/18/midlet-jar-signing-a-tutorial-revised/