Are you getting your text out of a database? If so, can you try using the actual accented characters rather than the HTML entities? If you have your encoding correct as ISO-8859-1, then letters such as û, é, è etc. should OK.
The problem is that the character entities were made for HTML, not XML - the early Netscape RSS specification had a doctype which declared a bunch of character entities (and so they are valid in such a context), but other RSS versions have no doctype and so no entities.
You say also that you are using RSS 1.0? That's a pretty rare beast these days with RDF syntax. You might be better off using RSS 2.0 which is XML-based, or if you can't change the character entities and have only basic needs, try a Netscape 0.91 format:
<?php header('Content-type: text/xml; charset=ISO-8859-1'); echo '<?xml version="1.0" encoding="ISO-8859-1"?>';?> [b]<!DOCTYPE rss PUBLIC "-//Netscape Communications//DTD RSS 0.91//EN" "http://my.netscape.com/publish/formats/rss-0.91.dtd">[/b] <rss version="0.91"> <channel> <title>Feed title</title> <link>http://www.example.com/</link> <description>Feed description</description> <lastBuildDate><?php echo gmdate( "D, d M Y H:i:s", getlastmod());?> GMT</lastBuildDate> <language>fr</language>