I had this issue with the 460 when some how Ambient Occlusion got enabled in the Nvidia control panel for guild wars 2. Might want to look at the controls and see what option got enabled.
http://www.geforce.com/whats-new/articles/nvidia-geforce-306-02-beta-drivers-released/
“At this time we can also confirm that the Ambient Occlusion profile added to the game yesterday can result in certain pieces of terrain becoming transparent. If you experience this issue, simply turn off Ambient Occlusion in the Guild Wars 2 NVIDIA Control Panel profile.”