Hi Heiko,

You wrote:
> I am interested in a mapping Unicode to LICR, therefore I should
> understand what a LICR really is.

I don't know if Ross Moore is on this list (latex-l), but he's also  
been considering such a thing for XeTeX; see the xunicode package  
(it's not on CTAN, oops:

   <http://scripts.sil.org/cms/scripts/render_download.php? 
site_id=nrsi&format=file&media_id=xetex_xunicode05&filename=xunicode.sty 
.zip)> ,

not a very succinct link, I'm afraid)

At this stage it doesn't cover very much of unicode, since with  
XeTeX's unicode input and output, it's debatable whether LICRs are  
the right way forward when you're talking about tens of thousands of  
characters...(well, such remarks have been made in the past, as I  
recall).

I don't know if the package will be helpful for you, but it might be  
interesting.

Best regards,

Will Robertson