I found a very strange bug that occurs when I launch Minecraft 1.5.1 or 1.5.2 from the new launcher (0.8.1) : in window mode, the "VSync" option is ignored (my fps are up to 120 even with VSync enabled, while my screen works at 60Hz), but in fullscreen mode VSync limits fps to 30 instead of 60 (so half of my screen frequency).

I have this problem neither when I launch the game with the old launcher nor with the latest snapshots (13w16a to 13w21a).