The question of what may or may not interfere with Airport and Bluetooth signals isn't so much about how close an object is to the top of the mini, but what that object is.
The higher the frequency of the radio transmission, the shorter the transmission range tends to be, the easier it is to absorb its energy in less and less dense structures, and the more prone to interference it will tend to be.
The more dense the object in close proximity to the mini's top panel, the more likely it is to absorb the RF energy. If it contains electronics components or metal, the more likely it is to interfere with Airport or Bluetooth signals. However, since Airport and Bluetooth operate at 2.4GHz, the wavelength is short, even small movements of the transmitter or receiver can make significant differences to quality of reception, particularly in situations where there is interference to the signal rather than absorbtion of it.