I'm not experienced in the high end raid scene, so my information might be slightly off, but my understanding is that right now, dual wield is almost always better for comparable-difficulty-to-obtain weapons (assuming you aren't tanking with Whirlwind, eating a damage shield, or eating enrage).
With the patch that adds in triple attack and buffs the damage bonus on 2HB weapons, I believe that will switch such that 2HBs are almost always better for comparable-difficulty-to-obtain weapons (assuming you don't need faster attacks for push).
I made a spreadsheet with some common weapons (there's a separate tab for Dual Wield weapons vs 2HB weapons) that, I think, correctly captures the current damage formulae:
https://docs.google.com/spreadsheets...nxk/edit#gid=0 (though it leaves out double attack, haste, bonus from strength, etc, since I think those things should apply proportionally equally regardless of the weapon).
Some examples from that spreadsheet (note that units are relative and don't directly correspond to DPS): BiS DW (Gharn/Fist of Nature/Baton of Flame) is around 35, and BiS 2HB (Abashi's Rod) is around 30. For context, Epic Fist + SoS is 27. Dual priceless / primal 1HBs is 28.5. NToV monk dual wield weapons should out-damage Abashi's too.
You may also be interested in Raev's analysis on a similar thread:
http://www.project1999.com/forums/sh...2&postcount=14