When does a jalapeño begin to produce heat/capsaicin?
The reason the 'seeds' you ate weren't hot is because they're not seeds. What you thought are the seeds in the pepper flower are its anthers. The anthers are the pollen bearing parts of a flower. The unfertilized, and therefore undeveloped, seeds would be found in the flower's ovary which is at the base of the petals. The parts of a flower are labeled in this diagram. 



I've tried both the seeds and the flesh of jalapeño peppers and noted the 'heat' in both but personally, I found the seeds a little hotter.