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There is no official information about Theros Cards yet. However, one of the articles in the preview week for M14 revealed there will be a cycle of dual lands in Theros (source: http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/ld/254) People at your gaming shop are speculating about what the cycle will be.
They'd have to be a cycle of rare dual lands. I don't see Wizards skipping duals in a core set, then not printing powerful or flexible rare duals in the next block. Then again, RTR block did some dangerous things to the quality of mana bases in Standard.
I was hoping they'd bring back cards like Nimbus Maze (I could even see that cycle working thematically with Theros's Greco-Roman flavor), but I doubt they would. Their logic behind not printing the check lands was that they were too powerful when combined with the shocks. The Nimbus Maze cycle would just rehash that problem and make it worse because those lands don't ever enter the battlefield tapped.
Each cycle of dual lands (since the Bayou era) has had some sort of restriction on it that balanced the fact that it tapped for more than one color. Selesnya Guildgate comes into play tapped, and Clifftop Retreat does the same unless you control the right basics/shocks, etc...
Assuming the dual lands in Theros will be a new cycle, they will also have some sort of new restriction on them that will either make them harder to use or less valuable to use.
I'd personally like to see something like:
T: Add (1) to your mana pool
T: Add RG to your mana pool. ~ Doesn't untap during your next untap step.
Seems like it could be potentially broken by turn 1 "ramping", but it would be fun nonetheless.