Better late than never: Notion about Relentless Rats forming an exception (alongside Shadowborn Apostle) to one-of-each-card has been added to highlandermagic.info
And Relentless Rats has been legal in singleton formats since the beginning of 2009.
See here: http://archive.wizards.com/magic/magazine/article.aspx?x=mtg/daily/feature/23c&page=5 (rule 402.8i)
And here: http://www.yawgatog.com/resources/rules-changes/ala-conflux/?id=13
And Relentless Rats has been legal in singleton formats since the beginning of 2009.
See here: http://archive.wizards.com/magic/magazine/article.aspx?x=mtg/daily/feature/23c&page=5 (rule 402.8i)
And here: http://www.yawgatog.com/resources/rules-changes/ala-conflux/?id=13