This bug affects Minecraft 1.6.1 and Minecraft 1.6.2 (not the previous versions).
Try to snap the Minecraft window on the right/left border of the screen on Windows 7/8 so that the Minecraft window takes half of the screen (this is a specific feature of Windows 7 and 8).
Now, go to another program and when yo go back to Minecraft, the window is no longer snapped and resumes its original positin and size.