Is it reasonable to assume Jar Launcher is set up to run jar files from Finder on OS X 10.5+? I have a jar file containing a swing application that needs to run on Windows, OS X and Linux. At the moment, Windows users are launching it from explorer, and Linux/OS X users are running a shell script to launch the jar.

However, I suspect the shell script is probably not necessary in OS X. I don't own an OS X machine myself, but Wikipedia says that Jar Launcher will run the jar file when it's double clicked.

Is it reasonable to assume this is always present on an OS X system with 10.5 or above?
The Apple supplied java implementation comes installed with Mac OS 10.5 and 10.6 and .jar files are associated with the jar launcher application. As long as the java application starts right into a swing interface it should open without any problems.

On Mac OS 10.7 Lion however the java runtime is not installed by default and users trying to launch .jar files will be greeted with a dialog asking them to install java to continue (which requires admin privileges).

Officially the apple supplied java runtime is depreciated so you can't count on it working in any future version of mac os x