Is it possible that was related to your Catalyst A.I. settings in your CCC menu? I've seen slightly similar artifacting due to the Catalyst A.I. setting for other games.
I never experienced this on my own system because I never installed the 12.2 drivers. People started reporting this bug when the pre-WHQL 12.2s came out, and that combined with a bunch of other bugs and it being a generally poor release prompted me to skip it entirely. I went straight from 12.1 to 12.3.
I can't say for sure about the CCC settings and whatnot, since again, never experienced it personally due to skipping the drivers. But given how many people I had to troubleshoot this for...the only common factor was the 12.2 driver set that came out at the beginning of March. (Granted I didn't exactly ask everyone for their CCC settings, but I saw it on many many 5xxx and 6xxx series cards, stocked and overclocked, etc.--the only people who didn't seem to have the gray pixels of doom were 7xxx series owners). It was a known issue when they came out and listed as much http://support.amd.com/us/kbarticles/Pages/AMDCatalystSoftwareSuiteVersion122ReleaseNotes.aspx by AMD.
Did you skip the 12.2 set, by any chance? That would be why you didn't see it.
Edit: Also, the picture I linked to isn't the worst presentation of it I've seen...just the only readily available link I had. That's pretty modest, actually, and usually gets worse in actual gameplay vs. the opening scenes. I've seen screenshots where entire lakes/rivers have turned into gray pixel wireframes, for instance.