Can any merchant process PIN based debit cards?

Yes. However three things must happen.
1. You need to have your processor set you up for the program- it is not automatic. A price sheet must be signed with the associated fees.
2. You need to get a pin pad to match your system. They start at about $75. It’s critical to get one that is triple DES, the new 2010 encryption standard or you are wasting money.
3. The pin pad must be ‘injected’. Only a handful of companies & locations in the US are authorized to do this. Essentially, the pin pad is married to the processor, the merchant id and the terminal make/model. The pin pad cannot then be unplugged and simply put on another system. There is a small fee for the injection, typically $25 wholesale.
Unless a customer already has a pinpad, the purchase is usually made from the same company as is doing the injection. The processor account rep usually handles the transaction on behalf of the merchant.  Merchants cannot buy direct that I am aware of.

This answer addresses card present transactions only.