I see they have engineered their own detectors [1], but could cellphones be used as EMP detectors? They have GPS and antennas after all. [1] http://www.blitzortung.org/Webpages/index.php?lang=en&page=3
http://www.digikey.com/product-highlights/us/en/ams-as3935-f...
The Blitzortung paper mentions that sensor isn't accurate enough to use to determine the distance of a strike, so I don't think a crowdsourced phone-based network could work particularly well:
http://www.blitzortung.org/Documents/TOA_Blitzortung_RED.pdf
I also suspect that a phone's RF frontend would be awful for lightning detection as it is tuned for 900+Mhz signals. You can read more about lightning's RF properties here:
http://assets.cambridge.org/052158/3276/sample/0521583276WS....
but it's clear that above 10Mhz you probably won't learn much of value about the lightning strike.
Elsewhere in this thread, an HN reader found that Nokia were working on lightning detection for phones, but it seems to use the FM radio receiver that was often built into SoCs, which could definitely tune itself to a much more useful frequency range.