My rss feeds in a Drupal install were showing an invalid character name error in Internet Explorer and were forcing a file download / save file dialogue in Firefox and others. I couldn't get them to display in-browser like they should have and couldn't figure out the issue!
 
I finally found a similar error here: http://drupal.org/node/312340   which ultimately was caused by a problem with rss feeds created by a module called Embedded Media Field which works with video / audio objects and treats them like node fields. Anyways, my rss feeds which were created with the Views module were including video nodes and that was the problem. Ignoring video nodes for my rss feeds fixed the problem immediately. If you have to include these nodes into your feed you can apply the patch found here: http://drupal.org/node/301741
 
If you found your way here then the keywords in this post helped out. I eventually ran into the posts myself but it just took way too long based upon the search words I could come up with...