I just updated from WordPress 1.6 to 2.13, and now all my RSS feeds display the posts alphabetically instead of in date order. This is uniform across all the feed types: atom, rss1, rss2, etc.
Here's an example: http://example.com/wp-rss.php
I've seen this problem intermittently over the years. Usually changing the feed URL cleaned it up. I'm using an old template -- I don't know if that would have anything to do with it or not.