Yum. Good one, sneff.
Note too that in the Thai language peppercorns are called prik thai, while chilis are called prik, plus usually some other descriptor which classifies the particular species of pepper. So the language reveals the origins. It's a testament to the Thai ability to incorporate and make something their own that chilis have become so integral to Thai cooking.
Those fiery hot little Thai chilis, by the way, are called prik kee noo (there's tones in there, but I can't write them in English), which translates as "mouse shit peppers". Because of their size and shape, or their ubiquity, I'm not sure.
However, I need to mention also that there are actually five flavours in Thai cooking: spicy, sour, sweet, and salty, as the able sneff mentioned, but also bitter. Sneff can of course be forgiven for leaving out the last one; it was certainly never one of my favourites. Perhaps most familiar to westerners who know Asian food is bitter melon, also used a lot in Chinese food (and sometimes on Iron Chef), but there are also bitter leaves and vegetables like pea eggplants. An ideal Thai meal - for a Thai - combines all the five flavours in a variety of dishes. For westerners, as I said, just the four sneff mentions are usually sufficient.