The whole issue is just goofy.
First of all, who in their right mind expects students to sit and listen to them for 50 minutes (or longer) without EVER wandering off mentally? It's unreasonable and goes against how we know the brain works. You can't fight nature!
Second, as Seth Godin notes, if your target audience isn't listening, it's not their fault but yours. Just like we tell K-12 teachers: classroom management (i.e., student attention) stems from good instruction.
Finally, these digital devices - particularly in conjunction with the Internet - are the most empowering things we humans have yet created. Why don't we start figuring out how to use them productively in class rather than banning them?
Our K-12 and higher ed students think we're totally clueless and they're right.
Be sure to check out Britt's post. I thought his scenarios were excellent.