FrequentSLF (and others talking about this aspect)
It's one thing to install Wi-Fi transceivers in an aircraft, and quite another when a consumer-grade product such as a laptop Wi-Fi transceiver is used aboard.
The former kit will be individually tested and be very carefully designed to have very low spurious (i.e. out-of-Wi-Fi-band) emissions.
A laptop might have poor spurious emissions because of a fault in assembly or manufacture, but as this is not tested for 100% of the production runs (it relies on type-approval of a small sample of units) there is ample opportunity for a problem like this to be limited to a single laptop and perhaps to the combination of that faulty laptop and an aircraft with a small defect in screening or grounding of the avionics, perhaps due to wear and tear or a maintenance error.
Note that Wi-Fi is usually not designed with power control of its transmitters (it's short range, so fixed output power is OK) whereas cellular phones are designed to have power steps to maximise the capacity of each cell and to reduce the variation in signal levels at a base station (essential for CDMA systems such as 3G).
The latter suggests that allowing indiscriminate use of Wi-Fi devices is riskier than allowing the use of mobile phones in the air.