Part of this is a repost from an earlier response.
When JRun 4 was first released, I downloaded and installed it (it did not abort on OS X), It seemed to work OK. Some months later, I downloaded a later version, but it wuld not unpack/install on OS X.
You could try the Linux version, using a procedure similar to the CFMX port.
Then, you would also need to port Linux CFMXJ2ee for JRun to run on Mac OS X.
AFAIK, there is a similar 30-day trial and then it/they revert to to a 2-IP developer version.
It would be a pretty cool system, if it works!
However, I suspect that some of the advanced JRun features are platform dependent, so you may need to manually deploy applications, and you may not be able to use things like clustering, failover, etc.
I have heard that FlashCom Server will likely be available on 'Nix platforms. But this does not mean it can be ported to Mac OS X. I am told that it is *not* written in Java, so binaries for other platforms will not run on Mac OS X -- pity!
Oracle and JRun on the same OS X box would be really cool!
These are all good questions. and it would be great to have a J2ee-compliant app server with the power of JRun 4 running on Mac OS X.
If you see a need for this, the best way to help make it happen is to submit a request to the Macromedia Wish List:
Every time I publish this URL, there is a surge of requests from Mac users -- and the MM people really notice.