Considering the point (pun fully intended) of Pinterest is for users to "pin" things like projects and recipes they'll never create to virtual boards, it makes sense that the logo incorporates a pin into the letter "P."
Sony's VAIO
At first all you see is modern typography, but there is actually a hidden meaning of Sony's VAIO logo. The first two letters represent an analog signal and the last two are the 1 and 0 of the digital world.