Apple has just published a page of podcasts tips that includes this gem on how to find the most popular programs by keyword...
"Although you could use the iTunes search box in the upper right-hand corner of the jukebox window, iTunes provides a search window specifically for podcasts in the left column. You can use it to search by podcast title, podcast authors, or by all podcasts. And once it returns a list of podcasts that meet your criteria, you order your results by popularity. Just control-click or right-click the column headers and select 'Popularity' to see the most popular podcasts that contain your search term."
Here's what this looks like...
