Actually, I have done something similar for my Android port. I
wrote a simple midlet manager:

http://davy.preuveneers.be/phoneme/p...droid_midp.png

If you click on a jad file in a file manager or on a jad link in a
browser, it will download and cache the corresponding jar file, add
a short cut in the midlet manager, and you would be able to run it
from there, or delete it, or ... without having to rely on shell scripts.

It doesn't create separate icons for each midlet in the main menu,
because in theory you can have multiple midlet applications in the
same midlet jad/jar file. I did this because I got these reports that
some people tried to delete one app, but noticed that a few others
stopped working as well. That is why everything goes through the
midlet manager, for which on Android there is an icon in the main
menu.

Another nice thing with the midlet manager is you can add a list of
urls to working midlets (like a midlet store), and leave it up to the
users whether they want to download and run it or not.

Unfortunately, this app is written in Java running on the Android
dalvik VM. For N900, this would have to rewritten completely from
scratch, for example in Qt4, but it would not be impossible.

There are two aspects you have to keep in mind though:
1) not all jar files are midlets, so I would argue to only associate
jad files to this app
2) error handling and reporting is a little bit less straightforward.
You have to make sure you can collect all the information you
need so that noobs can do bug reporting too. Otherwise, you get
eports like "my app does not work, can you fix it?". And this is not
really helpful.

Davy