Are publications on arxiv.org valued by graduate admissions committees?
"Publishing" on the arXiv is not a big achievement; actually, it's not an achievement at all.  In terms of how prestigious it is, it's not really any different from putting an article on your own personal website.  The arXiv isn't a publishing venue, it's a preprint server---it's purpose is to serve as a public repository for potential articles.  It has various benefits (it's probably more reliable than your own website, it gives credible evidence about when an article was posted and modified, which is sometimes desirable), but it achieves those in part by not refereeing the articles on it.

However, writing an article as an undergrad is certainly a meaningful achievement, and given that it can take a while for a paper to make its way to being in print, an appropriate unpublished article is still a positive for an applicant.  Of course, the admissions committee wants to distinguish a good article from a bad one, and since posting on the arXiv isn't a good proxy for quality, the committee will look for others (recommendation letters, for instance, or possibly just reading the article themselves).