I am having the same issue. Just like Christopher Wooddell, it only happens on one world. In single player, whenever I go from Nether to Overworld it crashes. I've found that if I open the world up to LAN and have a second player in the world, it does not crash for my player any longer. However, it will crash the server if the second player goes through the portal from Nether to Overworld.
I have checked my logs - I do not have any chunk errors. I am happy to upload my save if it helps.
Curt Hostetter
I am having the same issue. Just like Christopher Wooddell, it only happens on one world. In single player, whenever I go from Nether to Overworld it crashes. I've found that if I open the world up to LAN and have a second player in the world, it does not crash for my player any longer. However, it will crash the server if the second player goes through the portal from Nether to Overworld.
I have checked my logs - I do not have any chunk errors. I am happy to upload my save if it helps.
Edit (12/8/2013): Turns out I had a corrupted Nether chunk after all! I was able to fix it thanks to this tool here: https://github.com/Fenixin/Minecraft-Region-Fixer Very helpful!