I agree - the ideal way to deal with a customer who doesn't uphold his end of the deal by signing off on approvals is to bring proceedings to a halt until cooperation is re-established.
I am aware, however, of many situations where suppliers feel they have to suck the gas pipe with their customers or lose the business (and profit) to a supplier who is more accommodating.
It just "shows to go you" that customers don't really believe in following the letter of their own rules.
What do YOU say when somebody above you in the chain of command [the boss, sales rep, CFO, etc.] says, "Ship the darn thing and we'll settle up later. This customer isn't going to use the lack of THEIR signoff against us."
Even more important: Why do you say that?