What state do you live in? Do you do business through an entity, e.g. an LLC? If so, where is it registered?
PayPal is, varyingly, registered as some form of a money transmitter in many states [2]. While your federal protections are probably limited to antifraud, protections you're probably outside of (PayPal has good lawyers--you agreed to surrender lots of privileges when you opened an account), there may be state regulations you can use to, if not force action, encourage it.
Going forward, I tend to consider any business using PayPal for mission critical processes as being negligent with important risks. If you can't avoid using PayPal for certain lines of business, set up a nightly sweep from PayPal to a proper bank account.
[1] http://www.zdnet.com/article/fdic-decides-paypals-no-bank/
[2] http://law.bepress.com/cgi/viewcontent.cgi?article=1153&cont...
Disclaimer: I am not a lawyer. Only a lawyer can give you good legal advice. Don't take legal advice from Internet comments.