Like Capt Slash commented, if my Thai license isn't good enough to obtain the normal price, I'll leave and not enter the park. But, so far I've only encountered one park where they were adamant that I pay a farang price, and that was a few years ago. Like it was earlier stated, maybe entering on a bike does make a difference?
I did have one unusual occurrance when I visited the Kaeng Tana National Park, which is NE of Ubon Ratachatani. Upon stopping at the entrance, the guard came out and noting my dirt-bike and luggage, began asking where I'd come from. In my minimal Thai we chatted a few minutes as I told him of my travels. I then pulled out some Baht expecting to pay the Thai price. He refused to accept any payment and just waved me into the park. As I remounted the bike, a car containing a young Thai couple stopped behind me. The guard approached them and I saw him charge both of them to enter the park!!
Score one for 'we' farangs!