Every lender will require a pest inspection nowadays, and ultimately it is the buyer's responsibility. Pest Inspection is just another "due diligence" item that you as the buyer may want to check out during your due diligence period. This time frame should be spelled out in your contract and determines up until what date you can walk away from the deal, for any reason, and still get you earnest money back. Contracts as of January 2011 require not only earnest money but also some amount of "due diligence money", which is what you pay to have that specified amount of days to have the property inspected, surveyed, etc. You do not get the due diligence money back. That money goes to the seller for taking the property off the market for that specified amount of days. The amounts of each of these are negotiated between buyer and seller, but we typically see amounts as low as $100 for due diligence money on up, depending on what can be negotiated.
If the deal were to fall apart, somewhere during the process, unless it is specifically spelled out in additional provisions otherwise, the buyer is responsible for his own due diligence inspections, including the pest inspection.