If you JAR file have not JAD , you make with JADMaker.zip :
Download: http://www.geocities.com/mangokun/jadmaker/JADMaker.zip
Open folder :
Drag and drop JAR file to this window
I make test with StopWatch.jar RapidShare: 1-Click Webhosting
Open JAD file with Notepad :
Write: SEMC-StandbyApplication: Y
Click Alt + F4 on JAD window and :
Put JAR + JAD in phone and install JAD file , but NO start application , only click More > Wallpaper
OR
Menu > Settings > Display > Wallpaper > Application > and choose application
Here is Clock with edited JAD file to use as Wallpaper -> http://www.geocities.com/bongnate/JAR/Clock.zip