On modern (i.e. next-gen) consoles there is a significant performance penalty for using multiple streams but it really just depends on how you use it -- it's a very context specific and in some cases algorithm specific thing. Ex: If you're using multiple streams to do vertex blending for facial animation, it's faster to do that than the alternative of hundreds of bones transforming verts.

Also, it's just like Evil Steve said - depends on the GPU and driver set.