Is there any problem to subscribe an "Organ Donator Passport" resp. to have one’s organs donated in case of an untimely death, ch"v?

There various opinions about this issue. In a short answer the following should be appreciated; saving ones life by donating him a vital organ is a great Mitzvah of saving live – Pikuach Nefesh, (kidney etc.). Donating an organ that can help immensely another person is a great Mitzvah of Gemilut Chassadim (cornea etc.). On the other hand taking a vital organ from a man that Halachacly is not defined as dead is a terrible sin of murder. There's a big dispute among the Poskim how to define the moment of death. If you can assure that the doctors that will make the decisions of what organ to use and how, will operate only in conjunction with Halacha – you may sign such a document; but if that’s not the case, the donors' passport should not be signed. You should consult with a Halachic authority that is familiar with the legislation of your place.