Hi guys,
In brief, this is a little mod that can turn this

(tiny 20x24 image of the British Isles with test biomes)
into a monster 10000 x 12000 map (seed: Test World)

I explored this map with the help of mcexplore.py
Full size version : [link]
More Screenshots
HOW IT WORKS
Essentially, I managed to find the place where the minecraft worldgen decides what biome each area should be, and insert some extra code that gets it to read it from a file instead. I have no idea why it is so seamless.
The approximate default scaling factor is 1 map pixel to 256 minecraft tiles, but the world generation is quite noisy, so it doesn't quite match up exactly to the image supplied, depending on your seed.
Still, I feel I have achieved my goal of finally making it possible to create and play on 1:1 scale maps of the world.
All of the usual minecraft land features will be generated as per normal, including resources, mobs, villages, rivers, and the correct temperature and weather effects will be present as well.
Map color codes

forest, ocean
tundra, ice mountains
frozen ocean, taiga
swampland, mountains
desert, mycelium
plains, ocean
The colors are required to be EXACT. Therefore, no anti-aliasing, no feathering, no interpolation when scaling and certainly no photos/regular images. It won't crash, but you'll get a nice tundra world.
The EXACT RGB values:
oceancolor : (4,0,255) plainscolor : (6,255,0) desertcolor : (252,255,0) hillscolor : (146,157,142 forestcolor : (32,121,5) taigacolor : (15,213,159) swampcolor : (14,61,0) mushroomcolor : (240,0,255) iceoceancolor : (149,191,231) iceplainscolor : (207,255,219) icemountaincolor : (237,232,255)
[Example of a proper map]
ANY SIZE MAP IS PERMITTED, but I have no idea what will happen if you try to use a really big one.
The top left pixel of the image will correspond to the world origin. The calculations are such that the maps are infinite. The entire world will consist of various procedurally generated versions of your map repeated ad infinitum. However, the worlds will be mirror worlds depending on which quadrant of the world you are in. You can kind of see this in the full size map above.

It's worth repeating that these mirror worlds are all normally generated as well and are thus unique!!!!
The area where the maps are the right side up is the area with the positive x and z coordinates (when you press F3). This is useful when looking for land formations. You can also multiply 256 by the pixel coordinates from the top left of the image to figure out where you need to go to look for stuff.
CONFIGURATION HELP
The mod includes a .properties file to help you to set some options. Open it using any text editor.
ZOOM
This sets the scaling factor for the world generation. Minecraft defaults to 4 (1:256). The minimum is 2 (1:64). The maximum is 127, but it isn't recommended to try it.
RIVER_ZOOM
This set the reduction in the number of rivers. Use this when using lower than default zoom factors.
Warning: Using high zoom numbers will probably slow your world generation down.
OFFSETS
Use this to offset the center of the worlds. This diagram illustrates what I mean.

Use negative numbers to bring the correct orientation world closer to the spawn point. The numbers are in pixels. You can use much bigger numbers to move to one of the higher iteration repeat worlds.
ISLANDS
Minecraft generates random islands in the ocean. Use FALSE if you hate it and use TRUE if you love it.
Download the mod files:
[SP Files]
[SMP Files]
[Bukkit Plugin] (Craftbukkit only. The Bukkit port doesn't implement island removal, also there aren't any safety features to prevent it from messing with existing worlds at the moment. The map.png and the properties file is inside the plugin.jar)
Edit the mapsconfig.properties options.
Put it into the minecraft.jar, along with a properly formatted map image that is named "map.png" (similar to how the terrain.png is inside the .jar)
Delete the META-INF folder
CAUTION!!!!
THIS WILL AFFECT MAP GENERATION IN EXISTING WORLDS.
Changing the map image and generating on existing worlds will also have predictably bad results.
COMPATIBILITY
This mod is not compatible with any mod that adds new biomes.
Acknowledgments
This mod was made using MCP.
Changelog
v0.3
-Made the generation configurable with a .properties file. The proliferation of versions becomes redundant.
-Added a Bukkit port (no_islands doesn't work)
v0.2
-Fixed the problem of large water bodies appearing randomly
-Added a no islands version
-Added a 1:64 pixel to blocks version
-Fixed overabundance of rivers in 1:64 version
v0.1
-Release
THE INFINITE REGRESS
Suppose you had an image, then you use it to generate a minecraft world, then you use cartograph to generate a new image, and then you use it to generate another minecraft world, then ........
"It's turtles all the way down!!!"
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Seriously? Dude was last active December 4th...
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Secondly: You definitely are NOT allowed to post OTHER PEOPLES' mods behind your own AdFly links.
Thirdly: I'm not clicking those links, but imma guess you're distributing the minecraft.jar file too. That's flat-out illegal.
Fourthly: nope.avi
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I give it a 1.3/10 on the Troll Scale. Also, it's "momma" dipshit.
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