TL:DR; The reasons for doing this on any kind of general basis have to
be *EXCEPTIONALLY* compelling to make a business case for it, apart from
any possible legal ramifications.
I used asterisks *and* capital letters; that's about an order of magnitude.
Don't forget staffing.
Cheers,
-- jra
Hello,
I am looking for some guidance on full packet inspection at the ISP level.
Is there any regulations that prohibit or provide guidance on this?
Your better to discuss use cases than technology.
E.g. do you plan to do per-user behavioural targeted advertising?
To secure the network from DNS changer malware?
To block slammer worm?
To deploy a session border controller?
To deploy a carrier-grade NAT (LSN)?
To collect bank information and profit?
To enhance the QoS of VoIP?
To deploy a transparent web or video cache?
All of them use packet inspection. All can be achieved w/o packet inspection.
All of them vary wildly in how people would react 
So... phrase your question and 'guidance' around the use case, not the
method you plan to achieve it today.