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Party walks into a circular room 40' (foot) across and 9' high.  There are no other doors in the room.  The walls are actually a dozen 2 square wide walls, or a dodecagon (had to look that up, but it's easy to map), with etched lines going from the floor to the ceiling which has many circles etched into it.  Until the entire party enters the room, nothing happens.

As the last party member makes it's way into the room the trap is sprung.  With the sounds of sliding stone, the room drops 10' and the door disappears above.  As the room drops, a section of each wall remains revealing a glowing symbol on a metal plate in the wall below.  The etched circles in the ceiling do not drop either, opening as various size holes in the ceiling.  The party also notices there is a 1" (inch) gap between the floor and the wall.

The mechanics of this trap might seem complicated, but it just comes down to snakes coming in and snakes going out.  Regardless of the parties success or failure in the skill challenge to lower the room, the room will eventually empty and the party can continue.

This trap goes 5 times during one round.  For a five PC party, disperse the trap after each PC in Initiative or however makes sense for a different sized party.  The first four times the trap goes, 1' of snakes fills the room.  1' the first time, another 1' the second time, for a total of 4' each round.  On the fifth turn, or perhaps as the End of the Turn, a certain number of feet of snake leaves through the gap around the floor.  For each 1" of space, .5' of snake exit.  Killing the snakes doesn't change the trap, a dead snake takes up the same space as a live one.

Complexity 3 Skill Challenge, 8 successes before 3 failures.  Each of the glowing symbols on the metal plates is an access point to the trap, each wall can earn a success or failure toward the challenge once.  Thievery, Arcana or Religion Moderate DCs will earn a success toward the challenge and lower the room and open the gap another 1".  Each inch means another .5 feet of snakes leaving the room, so 8 successes and the initial 1" means 4.5' of snake leaves the room.  That is greater than the 4' of snakes entering each round so the room will eventually empty and the party can squeeze out the 9" gap around the floor.  After 8 successes, the remaining symbols fade to black and the room cannot be lowered further.  Failure at one access point causes all the symbols to flicker and that access point shoots sparks or oozes from then on.  Three failures, or failing the skill challenges causes all the symbols to fade out and the room will eventually fill with snakes, the sheer weight of which will cause the room to lower 1" each round.

The snakes are coming in 1' at a time, 4 times each round.  The snakes are leaving however many feet the gap allows once at the end of each round.  There are 12 places in the room the party can earn a success or failure toward the challenge which increases the speed at which the snakes leave.  So what do all those snakes mean?

As the party gets its bearing after the room has dropped, snakes begin to pour through the holes in the ceiling, the first few hit the floor and slither away through the gap but the number of snakes coming in is greater than the number of snakes getting out!

The first PC to go, has a pretty clear floor to move around on.  After the first PC, the trap goes so there is now a foot of snakes of various sizes and kinds slithering madly around the floor, a foot deep is a lot of snakes.  From 0'-3' of snakes is Difficult Terrain.  From 3'-6' of snakes is Difficult Terrain and the PCs are Slowed climbing through and over the snakes.  From 6'-9' of snakes, the party is Restrained, unable to move and barely able to act.  The room is only 9' high, so when there is 9'+ of snakes, PCs can do nothing.  Each turn another 1' of snakes would come into a room with 9' of snakes, each PC is moved 1 square clockwise toward the wall, caught in the current of writhing snakes.  At the end of a turn with a room 9' deep with snakes, the weight of the snakes lowers the room 1" before calculating the snakes that leave which is at most calculated from 9'.  This is a time element to the skill challenge, you can rule the challenge a failure if the room fills up or assign a partial failure depending on how the encounter goes. 

A Complexity 3 Skill Challenge is technically designed for the three PCs, so what is the rest of the party supposed to do?  Fight monster snakes, or course.  On the last turn of the trap, or at the end of each turn as the snakes are leaving, the party spots a particularly venomous snake enter the room or from one hole snakes stop coming for a second before a monstrous constrictor squeezes through.  Add these for flavor, if the party gets an early jump on the skill challenge, it shouldn't be too tough on them.  If not, don't have the monstrous snakes attack them while they are Restrained or worse, just don't give out XP for the snakes that get away.

The Water Filling Chamber trap from DMG2 doesn't get into the rules for drowning, but that's what the trap does.  Drowning in snakes is dumb, and Skill Challenges shouldn't be that deadly (most of the time at least).  So how you define Failure at the skill challenge or just getting completely overwhelemed by writhing slithering snakes is up to you and depends on the level of your party.  Humiliation is a good start, but how about loss of healing surges, maybe one for each time the party was completely covered in snakes - this will happen multiple times as the room fills up and empties if the party fails the challenge, don't let it kill anyone though.  Maybe you can emphasize how the snakes get into everything, maybe a PC loses a nonmagical boot moving at one point or a PC has to undo his pack to keep moving.  Then Failure results in some loss of equipment that gets swept away with the snakes.  Perhaps the snakes carry a disease the party is subjected to each time they are buried in snakes.  The PCs are getting out of the room either way, hopefully the players had fun during the encounter, there should be some reminder of their failure afterwards.