The search is your friend!
Just a few of the hundreds of threads regarding this issue, with much anecdotal evidence and several useful links to reports below.
Personally, I've witnessed the effect a mobile phone left switched on can have on aircraft instrumentation. Particularly the ADF. Observing a 15 to 20 degree swing as you're carrying out an NDB approach at minima is somewhat worrying I can tell you!
Anyway, links
http://www.pprune.org/forums/showthread.php?t=309578
http://www.pprune.org/forums/showthread.php?t=85447
http://www.airliners.net/discussions...ad.main/93818/
http://www.pprune.org/forums/showthread.php?t=237110
Should be plenty there to keep you amused.