Just came across an application named NameBench via a Lifehacker Article today. I ran it and it appears to be portable. It is a self-extracting app that runs off a temporary directory and cleans after itself once it closes.
From the site:
Application site: http://code.google.com/p/namebench/It hunts down the fastest DNS servers available for your computer to use. namebench runs a fair and thorough benchmark using your web browser history, tcpdump output, or standardized datasets in order to provide an individualized recommendation. namebench is completely free and does not modify your system in any way.