Blizzard explains "how" the ladder is reset...
http://www.battle.net/forums/thread.aspx?fn=d2-general&t=1023096&p=1&tmp=1#new1023096First it begins with the gathering of The Crystals of Everlasting Power. There are 5 of them, 3 of which are here on Earth (Rock, Air, Flame) and the other 2 are both in spirit worlds (the Ascension and Descension Crystals). Each of them have a specific color and clever name that plays off of what they stand for. The Flame Crystal is red for instance. Rock is green, Air is blue, Ascension is clear, Descension is black.
Obtaining each Crystal requires that a Blizzard Employee with a pure heart (also known as the Nonpareil) finish a task given by each Guardian who (obviously) guard each Crystal. These Guardians each have their own tasks, which again are thoughtfully related to the Crystal they protect. Using the Flame Crystal as an example again, the Flame Crystal Guardian requires the Nonpareil to reach into a mound of red hot coals to extract a helmet made of Wyvern scales. The helmet is worn and grants the wearer immunity to heat and fire. The Nonpareil can now accept the Flame Crystal which is in fact hot enough to render flesh from bone.
After collecting each of The Crystals they must then be combined at The Ivory Temple. Each Crystal is in fact not completely pure as the Guardians that protect them, being not from this world, slowly corrupt The Crystals. The Temple is used in conjunction with an elaborate ceremony including chanting, candles, and some cool shining lines that connect The Crystals half way through the chanting, that eventually cleanse The Crystals.
After The Crystals have been cleansed they must then be combined. The Crystals interlock with no discernible way to do so. Once the final piece is put in place a slight but satisfying 'shnnk' sound is made as if some sort of spring loaded latch existed, which it doesn't.
The final crystal, The Chat Gem is placed into the middle and a bright light emanates from the crystal construction as it begins to float. Loud choir music begins to 'Ahhhhhhhhhh', The Crystals explode as sparks rain down. At that point the Ladder resets and all Ladder characters are moved to non-Ladder.
The 'hold-up' so to speak, is that the Nonpareil has yet to be found. To date we have sent 28 Blizzard Employees to their agonizing deaths to attempt the ritual. None of which were pure of heart. However we will continue our attempts until successful. Once the 5 crystals are collected the reset will be announced to give fair warning before the final combination commences.