Ynet is reporting that Rav Avraham Sherman, of the Chief Rabbinical Court, has issued a ruling stating that someone who is deaf cannot convert to Judaism. The reason for the ruling is the idea that the deaf cannot keep the mitzvos, they also are incapable of accepting upon themselves the obligation of keeping the mitzvos.
The idea that a deaf person cannot (or is not required to) keep the mitzvos is not new -- it goes back at least as far as the times of the Talmud. Back in those days, there was no way to educate someone who was deaf. Sign language did not exist, nor did modern methods of teaching the disabled. While they certainly had the mental faculties (absent any other defects) to understand right and wrong, there was no reliable way of teaching them and informing them what the Torah expects of us. As a result, the deaf were truly marginalized in society and could not really be expected to be able to properly observe the mitzvos.
However, that is not the case today. In today's world, the deaf can be taught to be productive members of society. Today deaf children can be taught Torah and can observe the mitzvos. With current educational techniques, deafness no longer has to mean a life of isolation. The reasons to exclude a deaf person from the obligations of mitzvah observance no longer apply. Today we have deaf boys becoming Bar Mitzvah -- accepting upon themselves the obligation of mitzvah-observance. Would Rav Sherman tell a deaf boy of fifteen that he doesn't have to put on tefillin today because he's exempt? Would he tell a deaf boy not to bother eating matzah on Pesach? Would he tell the parents of a deaf boy to put the kid in public school since he's not obligated to learn Torah?* I highly doubt it. Jewish-born deaf people today are held to the same standard as the hearing. And that being the case, then why shouldn't the deaf be capable of accepting observance of the mitzvos? If the sole reason to bar the deaf is because they are considered incompetent (like the mentally disabled), then there is no reason to bar them today, as they are now capable of being full members of society.
* Assuming, of course, there is a yeshiva locally capable of catering to the child's educational needs.