The iPhone specific URL is http://reader.mac.com/mobile/v1/http://rss.slashdot.org/Slashdot/slashdot .
I was curious to about the source code of the page, really just to see if there was any interesting markup. So I tried to view the URL in Firefox... no dice. I was shown a page saying, "This Application Is Viewable Only On iPhone".
Hmm... it was time to cheat a little then. I googled around for the User Agent that the iPhone sends, opened a terminal and entered...
# curl -A "Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A543a Safari/419.3 " "http://reader.mac.com/mobile/v1/http://rss.slashdot.org/Slashdot/slashdot"
I can now take a look at what makes an iPhone/iPod specific feed :)