A few more thoughts:

1. It's quite interesting issue since it'll be a solution for modules<->subdomains for apache.

2. XOOPS is not just telling me that the "module don't exist" , but also logs me out in the sub domain (I was loged in in the main domain)

3. there is probably a simple solution with mod_rewrite without changing the mainfile code

4. I once saw it done with XOOPS (module in subdomain), but I don't remember the place nor the code

5. I kinda tired of all this (almost a week with no solution) ... I think I'll look for another solution (not XOOPS based)

tzook, Hi again!
The Gijoe way does not intefere with relative urls, your way does. If what you are trying to do was as easy as adding some lines of code than XOOPS would have friendly urls a long time ago.
Even if you manage to get the subdomain pointing to a module you would get corrupt links/forms all over your site and you could not navigate in it.
All links should be absolutized using that obstart() function and a lot of ereg replace. I´m going crazy trying to build an Url rewriting module for xoops, I just can tell you that there is no easy way to do it.

...Well, I might be wrong :)

I know about the links , but I'll really use xoops's abilities just in the main www domain, the subs will have just one module for each ... with no need for any XOOPS theme/user interation/system-wide-link , etc' ...
so the links issue is not buthering me too much ... just the fact that I get "module don't exist" and how to solve it with no security hazards...

So a "bit hacked" JIJOE solution might fit here ...
He turned www.your-domain/modules/_whatever-/
into www.your-domain/md/_whatever-/ ... perfectly.

I need it to be:
www.your-domain/modules/_whatever-/ ==> _whatever-.your-domain/

I can build or (hack existing) mini CMS for my needs, but I thought it would be nicer with XOOPS (for the back-office + the main domain)

anyway, my time is getting short, so I need to start working (planning) this mini-cms ....