Sorry but I think this is a pretty terrible idea.
Code on Stack Overflow, Server Fault, etc. isn't "code" in the typical sense of licensing where you're concerned about distributing it for use by others. It is an example of one way (but not the only way) in which a particular problem presented in the question can be solved.
It is more akin to code snippets in a textbook than a software project on github.
CC-BY-SA is (IMHO) the most appropriate license for code posted to a Stack Exchange site: We're licensing the documentation (including snippets/examples) to you. If you happen to cut-and-paste bits of it into your production environment that's fine - my understanding has always been that you should attribute it the same way you would if you posted it to another site (by a URL pointing to the source) and you're good to go.
We (contributors) are not providing any kind of warranty or statement of suitability for a particular purpose with our code snippets, or any other part of our answers. If you want to go ahead and make language to that effect prominent in the terms of service by all means do so, but I don't think muddying up the license terms for site content makes sense!
That said, if you feel we must have a separate license for "code" posted to the site taking an existing license with specific requirements and saying in your terms of service "…but it's OK to ignore requirements X, Y, and Z" is REALLY TERRIBLE -- you're creating two legal instruments (Terms of Service for the site and a License for Use/Distribution of the code posted therein) with conflicting requirements - this is the kind of stuff lawyers charge hundreds of dollars an hour to bloviate about in front of judges.
artofcode already spoke about the dangers of "Crayon Licenses"artofcode already spoke about the dangers of "Crayon Licenses", and I think those points are well made, so I won't reiterate their points here.
Suffice it to say I think you should either find a license without a notice preservation/license reproduction requirement, create one without that requirement (dear $_DEITY, another freakin' Open Source License! Kill me now!), or craft one specifically for Stack Exchange code snippets that describes precisely what attribution is necessary (e.g. a link to the page you got the code from) and what notice preservation is required, if any.