Descendents of Prophet Muhammad exist

From Shiah perspective:
http://hammorabi.blogspot.com/Imam%20Hussein/Imam%20Hussein.html

More neutral view:
http://answers.yahoo.com/question/index?qid=20071202064612AA0aY6K

I was told my religious teachers who are of the Shafie Mazab that
there are no living descendent of Prophet Muhammad because both Hassan
and Hussin, his grandchildren died. Initially I thought it was at
Karbala but the heads that were recorded appear to be only one but
verbally it appeared to be two because both deaths were narrated
together.

The descendent of Hassan call themselves Shariff and they should be
the most numerous because not many of them were killed. The descendent
of Hussin have only one surviving son but not sure if he survived the
trip to captivity but some daughters may survive because they were
taken care of by Zainab, their aunty.

Our religious teachers appear to discourage looking up to descendants
of Prophet Muhammad for fear of Shirik as what the Shiah had suffered
in revering the descendants of Ali. However pious Muslims should not
lie or hide the truth.

One possible reason was that these teachers were bribed and threatened
by the existing Muslim rulers who tend to form dynasties, not to
emphasise Prophet Muhammad's descendants in choosing leaders. It is
these fear that led to many of them to hide their ancestry while
preserving their titles of Sharif and Syed.

In South East Asia, they were revered not as leaders but as pious
people whose graves can give good luck. Their tombs were filled with
yellow clothes in order to indicate their holiness. In Sabah, most of
these have the Shariff titles.

If we are really concerned, we can do DNA tests but these tests cannot
really determine correctly complete genertic link. These people who
keep their Shariff and Syed titles are not the only ones who are
descendent of Prophet Muhammad. Since holders of these titles are
those who descend from the males line, DNA test can be carried out
easily and cheaply. Imposters can be easily discounted.

It is important for us to establish these genetic links not because we
want to exalt them but to pay our respects to Prophet Muhammad and to
confirm ourselves that Prophet Muhammad is indeed human with real
families. Those who pretend that Prophet Muhammad did not have any
surviving descendant is only trying to deify him which is strictly
forbidden in Islam. This is the same sin committed by Christians who
believe that Jesus does not have any descendants and therefore divine.