Advanced Custom Permalink Structure

I love WordPress — I’ve only once stopped using it at which point I desperately crawled back after becoming fed up with Movable Type, but I’m currently in a bit of a bind. I’ve scoured the WordPress code looking for a way to manually change the permalink structure from something like: http://www.website.com/2007/09/01/the-name-of-the-post/

to something such as: http://www.website.com/entry/the_name_of_the_post/

I can get as far as changing the structure of the URL, but as for the way the URL is generated with hyphens, I’m stumped. I would much rather have underscores in the place of the hyphens. I know that I can manually do this on each post, but that’s counter-productive and a bit annoying. Is there any way that I can possibly change this?

Thanks so much!

-Richard