I've heard they're doing the new dual lands any one of a number of ways. I'll just use U/R as an example color set.
#1: Split Lands
Land W/ Land X
When ~ comes into play, choose W or X. ~ counts as that card.
X: Add R to your mana pool.
Y: Add U to your mana pool.
Problem - A rules nightmare, cheating like that is way too easy ('Oh no, I chose X when it came into play, not Y')
#2: Bounce Lands
Land Y
T: Add 1 to your mana pool.
T: Add R to your mana pool, return Land Z to owner's hand.
T: Add U to your mana pool, return Land Z to owner's hand.
Problem - Nothing major, just wacky stuff could happen with Fastbond...
#3: Search Lands
Land Z
T, sacrifice Land Z: Search your library for a Mountain or Island card and put that card into play (un?)tapped. Then shuffle your library.
Problem - Better than allied search lands from Mirage.