Mark Pilgrim raises the inevitable question about ill-formed RSS and how to deal with it.  Mark offers parse-at-all-cost as a solution.  I think this problem can be solved completely if:

1. RSS feed proxy services with 'tidy' (parse-at-all-cost) and occasional validaton service becomes common place allowing either the feed producer or the consumer to deal with ill-formed RSS.
2. Encourage development and use of RSS/XML writer libraries instead of writing out tags and contents directly.