What did you see that lead you to think that the offer might not be valid? This offer appears on American Express's own website, and I don't see any expiration date, but I haven't combed the fine print thoroughly.

If you already have an American Express account, and if you are concerned, I would use the secure email facility on the website to request written verification that the offer is still valid. If the initial reply is that the offer has expired, ask why it is still running on their site and where the expiration date is disclosed.