Is 2-Weapon Spec (Fist/Sword) Dead?

Short answer: Kinda. It depends on itemization.

Combat raiding rogues have long had the option of combining a pair of weapon specialization talents when they couldn't get two viable weapons of the same type. Of these, MH Fist/OH Sword was the most powerful because offhand sword attacks proc mainhand fist swings. The most common spec for this purpose is this, though it was also effective for some to spec this way and change to a 4s/5r cycle that didn't need the extra SnD time (notice the only difference between the specs is one point moved from SnD to Sword Spec).

In 3.1, with the information we currently have, dual weapon specs are at risk. The patch will change Lightning Reflexes to a 3 point talent that grants an additional 10% melee haste. White damage currently makes up about half of my overall damage, making this point for point an extremely powerful talent, granting nearly a 5% damage increase for a mere 3 points (and 6% dodge on top of that). You can't turn it down.

The problem is that the rest of the tree is remaining the same, meaning that tight spec that could afford 4 or 5 points in Swords can now only afford 1 or 2. If there are no further changes, in 90% of cases you will be better off equipping a fist/dagger combo and going 5/5 Close Quarters Combat or getting 2 swords and going 5/5 Sword Spec, leaving you with one floater point for utility. Combo specs are effectively dead.

However, there are cases where the second spec I listed above can be modified to put only 2 points in Sword Spec and come out ahead: if you have Hailstorm but haven't yet gotten, or can't get, a Webbed Death.

According to the pre-3.1 version of Vulajin's spreadsheet, at my current gear level (somewhere between heroic and valorous), a Hailstorm with 2/5 Sword Spec beats all other offhands except Webbed Death. Webbed Death is only about 20 dps ahead (which for me is only abot 1/2 a percent).

Hand of Nerub and Sinister Revenge are too slow to compete (losing both poison and Combat Potency procs), while Murder ranks very close to Hailstorm, a mere 10 dps behind.

However, keep in mind this does not include the 10% haste as a factor, nor any other potential 3.1 changes such as glyphs or poison proc normalization. It remains to be seen how this will shake out, especially with Ulduar's itemization still unknown. But for now, it looks like an offhand sword may still be viable if you happen to have a better sword available than an offhand dagger. At least you aren't stuck waiting to roll against every other rogue for a Webbed Death before you can at least be competitive.