I'm not in that position, but I would have to personally avoid being the board member - especially the treasurer. No matter how open, no matter how many disclosures, no matter the intent, *someone* will take issue with it - and probably not to your face. Being a board member is tough enough with all of the questions and concerns surrounding business practices, and if you tack on even one possible conflict, it could turn ugly, fast.
Of course a lot depends on school, the PTO relationship as a whole and of that with the school. However, I think at some point we have all had "issues" with our fundraising companies in some form or fashion - whether it be delivery, or products, or payment, or... Do you really want to be in the middle?
I think it would also depend on how long you plan to be involved. If your child is in k-1, and if something, heaven forbid, goes wrong, whether it's within your husband's power or not, it's that much longer you'll have to deal with it. However, if your child's in 4th or 5th, then this may just be a great opportunity.
Our school is just this year switching to a fundraising rep that happens to be one of our parents, and I'm very excited at what she has to offer. However, I'm cautious by nature and although I know the truth because I'm on the board, if that parent were on the board, as a layperson, I would immediately think that the contract was obtained because of the relationship.
If you're a small (<200) tight knit school, it may not be an issue, but I would be afraid with anything larger, that you will absolutely have those parents that don't attend meetings and only have half of the details, that can ruin in for the rest.