It was really annoying to watch stem cell therapy get so heavily oversold in the 06 election cycle. Stem cell therapy seems to me like a fantastic way to give people cancer, and it turns out (registration required) that it is, at least in a recent case. Cancer is when your cells decide they're just going to keep reproducing past what the rest of your body can stand, and messing with the signals your body long ago relied on to stop making more of you strikes me as pretty dangerous - especially in the case of brain tissue.
Basically, if your brain keeps growing when you're out of the womb, your brain is growing cancer. A lot of brain cancer occurs in very young children, when the process of stopping the proliferation of brain tissue is critical, and prone to error. Introducing non-native cells that will proliferate according to signals out of sync with what is going on in the rest of your body strikes me as extremely dangerous. When the rest of your body is letting your cerebral tissue degenerate, what is in-sync is already quite dangerous, but trading a degenerative disease for a malignant cancerous one doesn't make things much better.
But hey, I can't blame people for trying. Or give up on the idea after seeing a case report.