There are still glitches that can corrupt a world's worth of work.
When I build too much stuff in single player, the world gets to be too safe and I move on to a new one anyway. I've only had the world get broken once. Then I started doing backups of them.
There are also glitches that can reset individual 16x16 chunks of the world, sometimes causing them to be different from the original seeded random area. In theory, the same seed always produces the same result, but glitches and the evolving nature of the world-generation routines produce some occasionally chaotic results, as seen in episodes 15-21 of the "let's play" at: http://www.youtube.com/user/MrRycraft#p/u/9/Mf9N81D_ElY