Use Feedburner instead of default feed on WordPress.com?

EDIT

You can not disable the default WordPress.com feed but you have an option:

If you have the css upgrade, add #feedarea {display:none;} to remove the default links. Use the feedburner widget code and add it to a text widget. Assign a class to it and using css absolute positioning move it to where the default links were.

My Original Answer below only applies to self hosted WordPress

Claim your feed URLs at Feedburner

Then add this to your .htaccess under #END WORDPRESS

RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /(feed|wp-atom|wp-feed|wp-rss|wp-rdf|wp-commentsrss)(.+)\ HTTP/ [NC,OR]
RewriteCond %{QUERY_STRING} ^feed [NC]
RewriteCond %{HTTP_USER_AGENT} !^(FeedBurner|FeedValidator) [NC]
RewriteRule .* http://feeds.feedburner.com/YourFeedburnerURL? [R=307,L]
RewriteRule ^comments/?.*$ http://feeds.feedburner.com/YourFeedburnerCommentsURL [L,R=302]