<content:encoded><![CDATA[<$MTEntryBody$>]]></content:encoded>
A resulting template might look like the following.
<?xml version="1.0" encoding="<$MTPublishCharset$>"?>
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
xmlns:admin="http://webns.net/mvcb/"
xmlns:cc="http://web.resource.org/cc/"
xmlns="http://purl.org/rss/1.0/"
xmlns:content="http://purl.org/rss/1.0/modules/content/">
<channel rdf:about="<$MTBlogURL$>">
<title><$MTBlogName encode_xml="1"$></title>
<link><$MTBlogURL$></link>
<description><$MTBlogDescription encode_xml="1"$></description>
<dc:language>en-us</dc:language>
<dc:creator></dc:creator>
<dc:date><MTEntries lastn="1"><$MTEntryDate format="%Y-%m-%dT%H:%M:%S" language="en"$><$MTBlogTimezone$></MTEntries></dc:date>
<admin:generatorAgent rdf:resource="http://www.movabletype.org/?v=<$MTVersion$>" />
<MTBlogIfCCLicense>
<cc:license rdf:resource="<$MTBlogCCLicenseURL$>" />
</MTBlogIfCCLicense>
<items>
<rdf:Seq><MTEntries lastn="15">
<rdf:li rdf:resource="<$MTEntryPermalink encode_xml="1"$>" />
</MTEntries></rdf:Seq>
</items>
</channel>
<MTEntries lastn="15">
<item rdf:about="<$MTEntryPermalink encode_xml="1"$>">
<title><$MTEntryTitle encode_xml="1"$></title>
<link><$MTEntryPermalink encode_xml="1"$></link>
<description><$MTEntryExcerpt encode_xml="1"$></description>
<dc:subject><$MTEntryCategory encode_xml="1"$></dc:subject>
<dc:creator><$MTEntryAuthor encode_xml="1"$></dc:creator>
<dc:date><$MTEntryDate format="%Y-%m-%dT%H:%M:%S" language="en"$><$MTBlogTimezone$></dc:date>
<content:encoded><![CDATA[<$MTEntryBody$>]]></content:encoded>
</item>
</MTEntries>
</rdf:RDF>
Note, I also added the content namespace declaration as an attribute in the rdf:RDF element.