Why switch the neutral?
As I interpret the National Electrical Code, the neutral wire is specifically NOT to be switched, fused, transferred or breakered in any way. It is to be grounded at the service entrance (usually the main panel) and electrically continuous everywhere.
73 de Jim, N2EY
As of NEC 2008 there are cases where the neutral is to be switched when dealing with certain types of generators feeding through transfer switches. Basically if the generator has the ground and neutral bonded together inside the generator, then the transfer switch used with that generator must switch the neutral along with the phases. If however the ground and neutral are not bonded within the generator, then the neutral is not switched. Pretty much all of the portable generators that you will find in big box stores etc. will have their grounds and neutrals bonded together, and are referred to as "separately derived systems".
The requirement to switching the neutral in this case is basically an extension of the notion that on a premises that there is to be one, and only one point in the system where the ground and neutral are bonded together. In the case of these portable generators with their grounds and neutrals bonded, and then being tied into the house electric service without switching the neutral you end up creating two points within the system where the grounds and neutral are tied together. Keep in mind that in all cases the electric service and generator grounds are always to be bonded.
This Alliant Energy doc spells out their adoption of the NEC 2008 code regarding generators and transfer switches:http://www.alliantenergy.com/wcm/groups/wcm_internet/@int/documents/document/012676.pdf