In pfSense 1.1 you will be able to let the pfSense resolve the URLs by adding an URL-Type Alias but for now (v1.0) you have to do it based on the IPs.
Even then, it'll be limited to what DNS returns. If yahoo (to continue the example) uses a DNS load balancer such as F5's 3DNS (now GTM) product, it's unlikely that two queries will result in the same answer. Using DNS to resolve hostnames can be useful and I can see the alias name being populated with a dns entry where there's a checkbox or such that allows pfsense to auto-populate the IP, I don't however, expect us to update the alias automagically.
–Bill