The problem with NK is its artillery placed along the DMZ. That alone would devastate Seoul where most of the population is held up, along with one of the most important cities for world markets. Not to mention you then have the issue of the mass Armour attacks which SK simply can't stop. You'd probably look at them pushed back to the Pusan permimiter again before any realistic counter-attack can be met. The US isn't station there in strength like it used to... there would need to be a big build up of military equipment and personel over a few months before they'd be ready to fight on equal terms. Public opinion is still subterrainian about armed conflict at the moment, especially with Iraq/Afghanistan... more troops are on their way to Afghanistan so to be honest along with Iraqi deployments and units resting on rotation I don't believe the US Army has the man power to make a stand on the Korean Peninsula. The USAF would be a big aid over there, but you would need nearly all their A-10's and god knows how many fighters to knock down the walls of MiG's... a chance for the F-22 to prove its worth perhaps? The DRPK has old aircraft, but it has lots of them... even MiG-21's are deadly if they get close enough...
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My only thoughts on a hypothetical attack would be this:
- Massive pre-emptive strikes on command and control systems, including telecommunications and any sattelite links they may have. This could be done using special forces on the ground, or a huge TLAM attack from off shore SSN's.
- The destruction of major bridges along the DMZ, it would create bottlenecks around the remaining ones and would severely restrict any ground offensive.
- Focused CAS/BAI to hit ground targets along the DMZ, priority targets would be the many artillery positions. These would have to be ID'd before and taken out by using guided munitions... I'm sure a squadron of B-2's filled with JDAM's would be great for this.
- Clear the skies... "blitzkrieg" style attack in the air, with simultatious SEAD/OCA strikes and fighter sweeps to gain air superiority, knock out their aircraft in the skies and prevent the rest from taking off by knocking out their runways. Most airfields have their hangers burried inside mountains so we would have to make some mighty big holes in their runways.
- Prepare for a ground offensive. After the skies are clear the armour would be at the mercy of the air force, the A-10 would make its presense felt here by knocking out tank columns. Whilst this is going on, the supply lines to the rear could be hit, NK has very limited supplies of fuel so this would be a cruicial point in the outcome of the war. Its shown in history once the skies are owned by the enemy advancing is incredibly difficult. Hopefully if the enemy advance can be stalled/halted then time could be bought for diplomatic ventures or to resupply/build up more forces in the region.
The above is based on the US/NATO/UN present in force in South Korea... SK on its own just couldn't hope to stop an all out attack.