I've already dug and 8x8 skylighted mineshaft down to the bedrock and will begin to create some horizontal shafts off of it, but right now I'm hurting for coal so badly I'm having to recycle torches like crazy.
I built a 5x5 sky-lighted mineshaft down to bedrock in my mountain fortress, which was a mistake because I was already 35 blocks above sea-level.
I spent a day mining down there, only completed around 1/5 of the bottom layer (of a planned 16) and found twice as much diamond as I found in 3 weeks of cave-searching. I'm scared to go back down there now due to the new monster-spawning rules which allows them to spawn in bright locations that far down.
Last night I ventured into the Nether for the first time, which is something I had put off until someone figured out what was causing the portal bug. It seems that when you create a portal in the main world, if the corresponding location in the Nether isn't "safe" (is in the wall or over a lava lake) then the portal will be created in the nearest available space in the Nether. But when you try to exit that portal, rather than linking back to your original portal in the main world, it will create a new portal in the corresponding location in the main world. Since 1 block in the Nether is equal to 8 blocks in the main world, you can exit the Nether in a location hundreds of blocks away, or in a dark cave some place. I've even heard horror stories of people exiting the Nether and ending up in a skeleton dungeon.
So last night I tested this out on peaceful, I created a portal in my base, entered the Nether, exited the Nether... and I was in a new portal in a random dark cave. I dug up to see where I was, and water flooded the area. It turns out I was in a small cave under the sea around 100 blocks south of my portal room. I went back to the Nether, and I could see that there was a wall to the north, so I dug into that for a couple of blocks, made a new portal, went through it, and I was on a beach around 30 blocks south of my portal room. I went back to the Nether, dug in another few blocks, created a third Nether portal, tested it, and it linked up to my portal room!
I destroyed the four extra portals (two in the main world and two in the Nether), and the result is that after building a portal, I now have more obsidian than I started out with.