This was on the Wizards site today at this address:
http://www.wizards.com/default.asp?x=mtgcom/askwizards/1107"The second exception is split cards. A split card on the stack has the mana cost (and name, color(s), etc.) of only whichever side you played—so Boom would cause Veiled Sentry to become a 3/3, while Bust would make it a 6/6. However, and this is where things get weird, a split card anywhere else has the mana cost of both sides. That doesn't mean they're added together—it means that any time you look at a split card's mana cost or converted mana cost, you get both answers. (The same is true for all other qualities of a split card that isn't on the stack.) Rough // Tumble‘s converted mana cost is 2 and 6. That means that if you (for instance) mill it away with Lammastide Weave, you'll gain 2 and 6 life, for a total of 8. But if a player plays Void and chooses 2, a Rough // Tumble in your hand will be discarded, because its converted mana cost is 2.
This is especially bizarre when clashing. If you play Oaken Brawler and reveal Kithkin Greatheart (converted mana cost 2) while I reveal Dead // Gone (converted mana cost 1 and 3), we both win the clash—we each revealed a card that had a higher converted mana cost. That means that your Oaken Brawler does get the +1/+1 counter, but I also draw a card off Sylvan Echoes. Everyone's a winner!"