Search Engine Technology Defeated by Underscore?

I just finished reading Gerry McGovern’s excellent book, Killer Web Content.
In it he recommends using hyphens rather than underscores to break up words in URLs for SEO purposes. This is because search engines don’t recognize underscores as spaces and so run all the words together into an unintelligible jumble.
It struck me that given how technologically advanced search engines are supposed to be these days, surely they have the ability to recognize an underscore as a space as easily as a hyphen?
Or have they just not got round to overcoming this particular obstacle?