There is an issue with the latest 306.02 Nvidia drivers that produces visual artifacts and transparent textures on some cards:
After experiencing this problem on my 560 Ti I went back to the previous 304.79 drivers and the problem went away, so the issue is definitely with those drivers.
I tweeted the issue to @Nvidia and @GuildWars2 although neither responded.
You may want to try the older drivers in case this is the problem you are experiencing.