It does seem obvious that companies, particularly but not exclusively in the IT industry, who restrict their employees access to the web in general, cut themselves off from awareness of competition. However, anyone whose worked in an office environment will be aware of the enormous drain on productivity net access can become. Hell, that extends to colleges, schools and everyday life as well. Corporate culture is probably important here, as an enabling workplace which values and promotes individual responsibility, and has mechanisms to track and reward it, will be in a better position to take advantage of web access.

In relation to your original question, companies are probably going to find it increasingly difficult to allow access to webmail (or even external mail accounts) for their employees during the day. As society becomes increasingly interconnected, and social networks more ubiquitous and mainstream, this creates an inevitable tension. Hell though, if works accept mandatory drug testing, and no fault dismissal laws, I should think cutting off employee access to personal email will quickly (if it hasn’t already) become an accepted corporate practice.