We're going through the same thing with our nearly-5-year-old ds. He talks about death constantly, mostly saying things like, "I wish I didn't have to ever die. I like being this person. I don't want to be someone else." (Ok, so the reincarnation thing has come up -- when he asked what happens when you die, I told him that no one really knows for sure but some people think you go to heaven and some people think your spirit is reborn into a new baby, etc....).
The most disturbing thing he's been doing is pretending to be dead. Every day now he says, "Mommy, pretend like I'm dead." And I tell him that I don't want to pretend something like that and he replies, "Ok. I'll pretend like I'm dead and you just cry." He also has funerals for his babies (his stuffed animals). I'm sure all of this is normal at this age, especially after reading the other posts on this thread. But it's still sometimes difficult to deal with. Especially because by ds bringing it up all the time, I'm forced to face my own fears about death!
Thanks to all you other moms who are posting here. It's nice to know that others are dealing with this issue too.