I am definitly in disagreement about 1 point here. When to pay. I've been a freelancer for 10+ years now and I will not deal with a customer who refuses to put some money up front. NOT ALL! There has to be some trust on both sides. BUT you must also have a contract of some sort. Never put all your money up front. Never turn over completed work until paid. Simple as that.
Now if you don't know if you trust them or not then use a service like
GetAFreelancer.com where you can put the money in escrow and pay it out only when the job is done and there is some arbitration ability since you can't just yank the money back either.
A few tips. Get references. Ask them for email/phone whatever of previous customers. Even if you don't check them, the fact that they are willing to give them out speaks volumes.
In this case, drop them like a hot rock. They should at least be telling you why there is a delay.