Apologies if this is not the right place to report this, but...
I got 1.6.2 via the new launcher today and since my multiplayer server was not yet up to date, I made a creative mode single player using the random seen 'rainbow connection' (since I saw on your pages a place that looked really neat under that seed' (using standard non-large biomes.)
relatively near the point of origin, I saw the attached screenshots... this came did NOT pre-exist the upgrade but was initialized and played entirely after 1.6.2 was installed... so this SEEMS to be a rendering flaw inherent to the game itself rather than confusion caused between versions. Anyhow, using that seed and using my machine stats, hopefully this can be reproduced quickly and analyzed.
Good luck guys... thanks for making this game... and curses! hehe, I spend more time playing than making my own rawr~