I know of no firm evidence indicating that baby talk is either helpful or detrimental in language development. From my own experience, however, I can say that infants learn from facial expressions and tone of voice as much as they do from words. They are learning these things long before they can imitate them.
Infants can detect differences in volume and tone earlier than they can discern words, especially when volume and tone are exaggerated (as in baby talk). I see no problem with it and have encountered no problems with it so far with my children.
However, baby talk is not the only form of communication we use with our children. While it gets the best reactions early on, we also talk to, sing to, and sign to our children at a very young age--younger than they can understand or mimic. As they grow older we adapt our communication to their maturity level, eventually cutting out the baby talk. This worked wonderfully for our first child, although it's too early to tell whether this is because of our method or whether we just got lucky with a bright kid!