RIM has stated that BB10 devices will require a service plan:
BlackBerry 10 Dev Alpha A and B – What Developers Need To Know «BlackBerry Developer Blog
Towards the bottom of that article, RIM specifically states that the Dev Alpha devices have access without BIS, but that release devices will require a service plan. BIS and BES are going to stay around. The only thing that isn't clear is if the PlayBook (wifi version) will require the service plan. I'm sure it would be a lot easier to have that requirement on the 4G models, since those go through carriers.