A report in TidBITS dated April 1 explains Seattle's recent coffeehouse ban: As we all know, Wi-Fi commonly operates in the 2.4 GHz band, the same band used by microwave ovens to heat food through the bipolar effect in which rapid switching causes friction in water molecules. (Not resonance as is often mistakenly stated.)
This can apparently lead to dangerous results not previously seen as the density of Wi-Fi usage increases. At a local coffeeshop, an ill-advised gateway placement and too many Wi-Fi users resulted in the espresso machine's boiler exploding from too much pressure. Several were injured, only one seriously; all were bloggers.
Seattle has formed the WTF (Wi-Fi Testing Foundation) to better understand this incident, but has proactively banned Wi-Fi until the results come in.