If that page doesn't exist, then according to specification, they don't and you can crawl any page.
Update: Some Yahoo! subdomains have 'em.
Yes:
http://search.yahoo.com/robots.txt
http://groups.yahoo.com/robots.txt
http://realestate.yahoo.com/robots.txt
No: http://maps.yahoo.com/robots.txt
http://omg.yahoo.com/robots.txt
http://yahoo.com/robots.txt Sorry, the page you requested was not found.