How do I open some nice looking UberChests?
Straight forward question. These chests are always locked for me, and having Lockpicks in the inventory doesn't seem to do any good.
Uberchests are unlocked by finding a lever somewhere on the same dungeon floor. Pull the lever, and the chest unlocks (here's the entry on the wiki that mentions them). Unfortunately, there's no way of knowing where that lever is, or whether or not it unlocked an uberchest when you pull it; all levers seem to say "You hear the sounds of machinery" when pulled.
Since levers almost never do anything bad, (except for the mass pitting mechanisms) there's really no reason not to pull every one you come across; in the course of clearing out the dungeon level, you'll probably stumble across the chest once again after you've found the appropriate lever.