ScienceCareers has a very good article on the importance of undergraduate research work, especially if one is looking to enroll in one of the competitive schools for graduate work. So if you are an undergraduate science major, you might want to read this.
I've emphasized the importance of this to physics majors in my So You Want To Be A Physicist essay. Such research work will tend to distinguish you from other graduate school applicants. It is certainly a very important item in your resume that can easily work in your favor. So it is something I strongly recommend on doing as an undergraduate.