The new borders appear in any world now at 30,000,000 blocks from spawn. So technically the maps are no longer infinite, they are just walling us out of the Far Lands. Which is freaking far away, I've never traveled more than 100,000 blocks from spawn in any of my worlds so far, and that took a very focused concentrated effort. But now at least custom maps with "small" world sizes can have borders that aren't ugly walls of bedrock, or even use custom border graphics.
So I messed around with the custom map stuff last night, I don't know what half of the expert parameters actually do but wow can you make some crazy unique worlds. Maps without oceans are actually kind of cool in an apocalyptic way, just deep gravel flatlands and valleys separating the biome continents with the occasional lava lake or ravine or mineshaft exposed. Worlds with smaller than normal biomes are very reminiscent of early beta worlds to me. Especially once you increase the height and noise stuff, then you start getting very crazy worlds. In contrast, making a superflat world with a ground level of 64 and tiny biomes is actually freaking awesome. That one is going to be my new "creative" experiment world.
Yeah, I'm going to be making a new single player world once again. My old one is a hodge podge of various areas I've wandered off to for each new update that added stuff, but with these custom settings I want to make something tailored a bit. Something with biome size taken down a notch, dungeon rarity and mountain height increased, larger rivers, and ore clumps increased from 8 max size to something like 12 or 16 max size. I don't need ores to be more frequent but I do want ore veins to be more robust. I want to be able to make extensive rail lines and use ore blocks as building materials. Basically I want a map very rich in ores with awesome unique terrain to explore and build in. Oh yeah, and tons of the new stones too.
So yeah, new world for me.