tedster - 6:33 pm on Feb 1, 2011 (gmt 0)
A robots.txt disallow rule stops spidering - but URLs can be indexed even though the content isn't crawled. The information listed can come from internal or external backlinks, DMOZ etc.
However, seeing the feed indexed on a Wordpress site seems odd. Are you seeing these URLs only for a site: operator result - or are they ranking for some kind of regular query?