If Nasdaq OMX group were to renege on their offer to buy e-Speed for $1.234 billion, their cred in the business would be dog poop. Look, guys like Howard Lutnick and Bob Greifeld don't make public announcements of deals, and then later change their minds and pull out. That isn't how they became the CEOs of corporations. The way they became CEOs is by having enough integrity to keep their agreements and by closing deals, not by backing out of deals.
Look, if you think this deal is going to fall through, then sell your shares and pocket a tidy profit. No one will blame you for cashing out.