Ok - Keep in mind that all of the suggestions in the thread might not make for a very fun game from the player's perspective. But, you are asking for advice on how to hand them their butts on a platter, so we need to focus on the strengths of the kobolds as well as do everything we can to focus on the weaknesses of the party.
First thing - Kobolds won't need light in their home. Don't have any light sources.
The actual lair should be a long way in. Many twisty passages. Passages should be rough with a lot of deep dips and they shoud be tall enough for kobolds, but not much else. For the first few hundred feet, the party wil have to crawl through something that feels downright claustraphobic. They shouldn't have line of sight of more than ~10 feet.
These passages should be guarded, but not by any guards in the entry tunnel. Have tunnels running around and over the entry tunnel. Cracks between the two provide a way for the kobolds to know where the party is, and sometimes they can even fire a crossbow through. This is very frustrating to the melee machines. Even if the damage is negligible, the fact that they are being attacked and can't respond is torturous.
Think in 3 dimensions. Why wouldn't kobolds have a 40' shaft up, 30' horizontal to a 50' shaft down? With a few rocks, they can make life uncomfortable for the people crawling through their tunnels. In one kobold lair my party went through, they got so accustomed to the ups and downs that they missed the secret door, found the down shaft and started climbing down. All the shafts were conveniently provided with ladders after all. This particular shaft was more than 100' down and the bottom had huge rusty spikes jutting out of the floor. The ladder was rigged to fail. The climb was long enough that adventurers, being impatient, decided to have multiple people climb down once it became apparent that the bottom was a long way down. Heh, that was fun. especially since they couldn't climb a convenient ladder back up.
Is there a water source? Remember those dips in the entry tunnels? Make sure two of them have a nice deep bend, like in your sink plumbing. Make sure the stretch of tunnel in between doesn't have any cracks. After the party enters, a small team of kobolds runs into the tunnel, through one of the ends, sets up a lot of firewood, sparks up a fire. Then, they retreat back they came from and start pumping water into the two deep bends. That section of corridor is now sealed and the fire will devour the oxygen. Now, when the party leaves, they have to swim through one of the bends, crawl through the airless section of tunnel on top of the embers of the fire, swim through another bend and then breathe. Sure, a few of them might be able to do it. The nifty thing is that if they figure out a way to fill that section with air, the fire will spark back up until it runs out of fuel.
Oil - lots of oil. Coupled with a few cherished flasks of Alchemist's fire, held for an emergency, you can do some damage.
The local equivelant of flypaper. When the party starts crawling on it, it sill stick the their hands, making it hard to hold a weapon properly. Tanglefoot bags are a fun addition if you are doing rolling boulders.
If the kobolds have a sorceror, you might consider making a few tunnel sections positively restrictive. Such as 2-1/2 wide. According to Page 76 of the 3.0 DMG, you need to have room to dodge to gain benefits for Evasion. The rogue will _hate_ that of a fireball goes off right there. Of course, the rest of the players may argue that they should get bonuses since the rogue is effectively acting as cover - sure, allow that.
The whole complex should make no sense at all to the players. This isn't a standard dungeon. This is a living warren. The kobolds probably spend a lot of their "spare time" improving defenses, creating dead ends, etc. Once the party gets out of the entry corridor, they should find so many tunnels branching in every direction that they are afraid to proceed. Thos tunnels should angle up, down, go straight, go everywhere. The lair is still there because it is too much of a nuisance to take the kobolds out. You don't want to give the party a stand up fight. You want to always attack one person at a time. This is a game of attrition. Make the party burn up all their healing spells. Put them in areas where they are always fighting with penalties, can't use their best weapons and can barely see what is happening. You are betting that a lair of kobolds has more HP than the party with all of their healing magic. Do everything you can to avoid letting the party attack more than a single kobold at any given time. How do you cleave when you can only see one of them? How do you flamestrike a bunch if there is never a clump? Fight dirty, the kobolds are in a win or die situation.
A well-prepped party of this level can still wreak havoc on the lair. They start by opening up the entry corridor and getting into the guard corridors. Start bypassing the little traps laid out for them. But, an ill-prepared party loaded up on offensive spells, will be less useful. They will be expecting to see that nice clump of kobolds that they can flamestrike. They will be looking for the cleave combos.
They won't get it ... until you want them to. When they are all out of spells, bruised from rocks dropped on them, swollen from the stings of a hundred scorpions and finally crawling into the Kobold Sorceror Kng's main chamber. The kobold sitting on the throne smiles stupidly as the barbarian finally stands up to his full height with that wicked look in this eye that means Rage-Charge-Power Attack. Then the 8th level Improved Invisible Sorceror, that is letting his stupid, halfwit jester sit on the throne, unleashes a lightning bolt through the party. The 5th level Improved Invisible Rogue unleashes a sneak attack and the 4th level Improved Invisible Fighter readies an action to set his spear against the barbarian if he charges the halfwit false king. So far, you have used all of the Sorceror's 4th level spells, and 1 of his 3rd level spells. Improved Invisiblity lasts 1 min/level. How many attacks can just three kobolds get in with that?
