It really depends on the style of music and what you are trying to achieve by micing them.
I usually mic them in the context of a studio orchestra. For that I'll use a single small diaphragm condenser microphone sort of centered over the set about 3-4 feet over the drums. My preferred mics are usually Neumann KM 140s or Schoeps CMC6-MK4. It should be noted, however, that the main pickup is with a microphone array going across the front of the orchestra. The timp mic is simply a spot microphone.
If I needed a more present sound for some sort of a pop recording, I would probably use 2 mics spaced over the 4 drums or perhaps 4 mics at a closer distance. Like a drum set, though, I usually consider the timpani a single instrument and have it mic'd to reflect that.