Thanks to RSSxl--"a free service which will create an RSS feed for any site based on the criteria you provide" (HT: Joe Carter)--Desiring God has added a couple of RSS feeds for John Piper's sermons, articles, and radio program:
http://www.desiringgod.org/feeds/sermons/
http://www.desiringgod.org/feeds/freshwords/
http://www.desiringgod.org/feeds/radio/
