No Edward, it won't hurt anything. Just make sure, as JoelHit said, to only use https or http for any given page. Most people use https for their cart and checkout pages, and http for everywhere else - but the important thing is to be consistent and redirect as necssary.
One minor note - https is slightly slower, so you could argue that would affect SEO since site performance is now being considered by SE's. But this is a minor concern.
