For the purpose of future discussions, I'd like to distinguish between stateful and stateless rss aggregation.

A stateless aggregator is one which just consumes and represents the current information in an rss feed (or set of rss feeds). It doesn't remember items that were previously on an RSS feed, and so is effectively just rendering the current state of the RSS XML data.

A stateful aggregator remembers any items that it picks up over time.

The Firefox live bookmark technology is an example of a stateless aggregator, as is the Planet aggregator software (which we use at work to aggregate our blogs into a single web page and feed).

Technorati, IceRocket and desktop rss aggregators like RSSBandit and Thunderbird are examples of stateful aggregators.