My friend Ian who is an evangelist for all things mobile has posted some comments on the impact that more connectivity has on extending working hours. I agree with Ian about the enslavement of employees by Blackberry. I would also add that the speed of response could also affect the quality of the response as some people feel compelled to respond as fast they recieve messages.
One of my business acquaintances compared his new Blackberry to having a crack habit, Blackberrys are even more socially disruptive than a mobile phone, as they respond to both calls and emails. To use a workaholic cliche, it is even easier to bring the laptop to the beach when it slips easily into your pocket.
When I use my Nokia smartphone, I have my emails download but do not have reply set up, thus I can satisfy my own curiousity, but still save train journeys for the valuable commodity of thinking time.