In Cataclysm it is likely that it will be a DPS increase for Enhancement Shaman over using the traditional two melee 1Hers and WF/FT. The reason we're forecasting this is because our Mastery (increases all Elemental damage done) causes the spell damage side our of DPS to scale very well in comparison to our melee damage. We can stack a lot of Mastery giving us a large multiplier on our spell damage while using a caster MH to greatly increase our spell damage. That's the problem in a nut shell. We'll find using a caster MH to be a DPS increase because our spell damage has such a potent modifier.
As far as solutions go; some Shaman have suggested we change our Mastery to allow it to also boost melee damage done. I don't tend to agree with that for several reasons I won't get into unless it comes up in discussion. I don't see our Mastery as the core of the problem. It's doing its job just fine; allowing our spell damage to scale better.
The problem is that while our spell damage has a strong scaling stat in Mastery our melee damage is decidedly lacking in scaling. For melee classes this normally occurs through talents that give a flat boost to melee damage done or as a multiplier on a melee stat like Strength or Agility. Enhancement has neither of these and I believe that is the root of the problem. Our spell damage has good scaling thanks to Mastery while our melee damage has no scaling at all.
To stem the tide of caster MHs looming on the horizon I propose adding some sort of melee damage scaling to one of our talents, Focused Strikes would be a good candidate. Focused Strikes currently doesn't contribute much to Enhancement in terms of a DPS gain. I would suggest adding this to the current talent;
"... and also increases all melee damage done by 5/10/15%"
With an effect like that Enhancement's melee damage would scale better and even out the imbalance between our spell damage scaling (from Mastery) and our melee damage scaling (of which we have none). The exact numbers would likely need to be tweaked, 5/10/15% is just something I came up with off the top of my head. A higher number might be needed. I don't have the data to come up with an exact figure right now.
In summary, because our Mastery allows our spell damage to scale so well Enhancement Shaman using caster MHs is looking like it will be superior to using the traditional double melee 1Hers again. To fix this I believe Enhancement needs some manner of melee damage scaling in their talent tree, of which we currently have none, to even out the scaling of our spell and melee damage.
Edit: Changed some things slightly to reflect that we would want to use a caster mainhand with WF and wouldn't necessarily want to use double FT again. Of course the problem is still there if we want caster weapons though, even if we weren't using double FT along with it.