If using a HD texturepack with 4096x4096 or larger cloud textures, the clouds are incorrectly rendered grayish-blue, cover the whole sky and have massive black stripes on them.

For example:
Using "clouds_2048_scuttles.png" (taken from Scuttles LB Photorealism), the clouds look as intended (apart from showing MC-8779, see screenshot "img1.jpg").
Using "clouds_4096_scuttles.png" (doubled resolution), the clouds aren't properly rendered anymore (see screenshot "img2").

EDIT:
Installing MCPatcher on Minecraft 1.5.1 fixes the problem, so it's not a hardware issue !