At MacRumors, I've been asked to elaborate on whether it's possible not to let users browse non-company Web pages, preferably with a mirrored, offline set of the company's pages for reference to get 3G data usage costs down.
Let's start with the former (to keep the size of the article down, I'll discuss the strictly offline solution in a separate writeup). Note that the solutions I present can be not only used in a corporate environment but, say, at home if you don't want your children to access certain “adult” pages.