Thanks to Russ
I discovered a problem in my feeds that caused Bloglines
(and probably a few others) to keep on marking every entry as new.
The comments counter is automatically incremented each time a new comment is added. Bloglines, in addition of the looking at the entry dates, also scan the actual text.
Per Russ suggestion the feeds now include a dynamically generated image to indicate the number of comments associated with an entry.
Sorry for the inconvenience.