On May 2, 2000, the CAA published a report entitled 'Interference Levels in Aircraft at Radio Frequencies used by Portable Telephones'. It could be downloaded from
www.srg.caa.co.ik/srg/srg_news.asp
It can probably still be found somewhere on the CAA site; its conclusion is that field strengths high enough to exceed the susceptability levels of some aircraft equipment can be found.
One problem is that as an aircraft gets older, you cannot be sure that all the cable screens will maintain their total integrity, so immunity can actually decrease.