I would not get so hung up about how close they are in frequency, closeness causes desensitisation or blocking, and equipment is usually designed to cope with this. What is harder to design to is protection against harmonic interferance, either direct or combined with additive/subtractive elements internally or externally. Taking a very simple case:
You want to listen to 121.5Mhz. In order to handle this frequency the radio needs it converting to an intermediate frequency, say 10.7Mhz. The radio needs to run an oscillator at 110.8Mhz which when combined with the required signal gives 10.7 Mhz (121.5 - 110.8 = 10.7). However, another frequency of 100.1 Mhz also gives 10.7Mhz (110.8Mhz - 100.1Mhz = 10.7 Mhz). Therefore as well as receiving 121.5 the radio will receive signals on 100.1Mhz (probably KISS FM in London UK).
THe above is over simplified to give clarity, techniques such as multiple intermediate frequencies etc can overcome the problem.
But now add in a mobile phone. These frequency hop whilst in use and use pulsed modulation (particularly good at producing rich harmonics) Therefore there is a good source of frequencies for any radio equipment to "mix" with causing problems.
In practice much mobile phone interference is also caused after the RF stages in the more poorly protected audio stages.
So whilst it is possible to design in protection it is easier to reduce the opportunity for it to occur (ie. switch the damn things off).