My first question to you is, what are the chances of someone writing a virus to infect SCADA systems? And then there was stuxnet....
As for your question, it is possible. The normal call back for this issue is the 2007 blackhat demonstration about hardware hacking. Here are a couple articles that hit on that demonstration... eweek, zdnet.
Frankly, when it comes to video cards in particular, i have no solid evidence either way. I would not think of it as a major threat at this time. Personally i have no doubt that video cards could be used for persistance, the only question is when will we find the first one in the wild?
Also, if anyone knows of an actual case of this in the wild, i would love to hear of it.
At the moment, i'm more afraid of a technology company being compromised that produces memory sticks than one that produces video cards. If you're looking for money, then you're going to go after the easiest route, and that seems like an easy choice at this point. If i remember correctly there was a sandisk incident similar to what i just mentioned, though i haven't done any research to back that up.