@Christian Burger We already know it is because of the raise in the default height level and the changes that had to be made to allow this increase in the first place.
More screenshots of the actual consequences of the bug:
http://minus.com/m9DDNSBhYwO0u
Can confirm that I have the issue as well on a linux fedora 17 machine, with 64 bit java 1.6 (IcedTea)
Allright, I think this is not a Minecraft client bug. After we looked in the plugin that handled this 'texture switching based on region', we found multiple registered listeners in the plugin that caused duplicate texture changing packets being sent to the client.
After preventing multiple listeners from registering, the clients I tested on switched more quickly.