One of the most debated topics in the SEO community (besides for the black hat versus white hat topics) is the question; do keywords in the URL improve a page's rankings? It is funny, some top notch SEOs claim they do, while others claim they do not. A thread at Search Engine Watch Forums named Do keywords in URL influence your rank? by a WebCEO employee, provides a comprehensive but questionable study on the importance of the keywords in the URLs.
In the study, they compared "competitive keywords" and "uncompetitive keywords". The conclusion of the study reported the following:
For Yahoo presence of keywords in URL is a considerable ranking factor.
In case of competitive keywords, Google takes presence of keywords in URL as overoptimization and penalizes a page.
In case of uncompetitive keywords, Google uses this factor in its ranking process.
Please make sure to read or skim the thread.