Wa alaykum salam.
What transgendered people tend to say is that they were born with the wrong body (i.e. their mind does not match what genitals they have). As muslims, we believe that God creates. He never makes mistakes, so to say that God gave you the wrong body is a huge insult [to God]. You are more than a mishmash of cells-- that's what the secular world doesn't understand. The Qur'an says that Allah azza wa jal created us in stages. He chose what sex we would be and He chose what the proper expression of gender is, irrespective of how that makes trans people feel.
Anyone who thinks they've been given the wrong body is committing blasphemy, because what they're saying is that Allah made a mistake (and we know that He never makes mistakes). Transgendered people need salvation just as much as anyone else, though, so let's not use their sin as an excuse to berate or insult them.