I don’t see this being a major problem for Thompson, because he’s in a field with McCain, as well as Giuliani and Romney, who all supported campaign finance reform in different ways. I think in McCain’s case it ended up being a big deal because it represented the beginning of his break with conservatives, which was later compounded by his failure to support the Bush tax cuts, his stance on immigration, etc.
What this does, if anything, is help other candidates with the “See, Thompson’s not really that conservative” narrative. So, if you liked Giuliani, McCain or Romney, but switched to Thompson because you got swept up in the hype that he was the second coming of Reagan, this may make you consider the other candidates again.
Ultimately, though, I don’t think individual issues are going to sink Thompson. As I’ve said before, his problem will be in explaining what he has accomplished in his life that would actually make him effective at being president.