I wondered the same thing, but now I see a search bar. Was I blind all along or did this just get put there recently?
I guess I was only blind for a week:
http://googlereader.blogspot.com/2007/09/we-found-it.html
It's new. They rolled out a few changes recently.
But why does this Google product not feature search??! I really need it in the reader, it makes no sense with no search...