I issued an erroneous report on the version 1.4 bug tracking page on Minecraft Wiki. I generated New Players' Guide Tutorial World III (named after the Paul Soares Jr. series of Survive & Thrive worlds) in 12w27a, which was given random seed -2613378232004986922. In Minecraft 1.4 (the pre-release), I generated a new world with the same seed and did a standard SSP build, but put my mine in a different location, starting from what I call the "spawn dugout" at X+267 Z+242 just south of the spawn chunk. It is a small, naturally occurring shelter with visible coal, there is a jungle to the east, mountains to the west, a desert to the north, and taiga not too far south, making it a very interesting free build seed with a ravine even closer than the nearest mountain visible from spawn in the northwest, it is an ideal first-time player's seed.
When doing the 1.4 test SSP build, I started an angled mine shaft (100% grade) leading east from the spawn dugout, and hit a very interesting cave system. I had already explored around to verify that world generation had not changed (later analysis showed slight changes. As a result of finding this cave system, I built the same mine shaft (also 100% grade) from Tutorial World III and hit no caves all the way down to bedrock. I had assumed that cave generation had changed without any note on the change list and made a note to that effect on the Minecraft Wiki bug tracking page, which is now locked.
I had planned to do a comparison of both maps using the Unmined 0.3.165 Anvil map viewer and just did so. It turns out that I had missed the roof of the cave I entered in the 1.4 test by just one or two blocks in Tutorial World III, likely because I started the angled shaft a few blocks further east. In Unmined, the cave systems appear to be identical. It was a big groaner to find this, and I apologize for the erroneous report.
That said, I have since noticed that the 1.4 map is shot through with a lot more abandoned mine branches in very different configurations from Tutorial World III, indicating either that a major change was made to the abandoned mine generation code between 12w27a and 1.4 (no documentation found on Minecraft Wiki), or that abandoned mine generation is unaffected by the seed. I will generate at least two more worlds with the seed in 1.4.2 for comparison in Unmined. If the former bears fruit, I will update this issue. If it turns out to be seed-independent, I will add a note on the Minecraft Wiki page [[Abandoned Mine Shaft]]. See you in a bit.