I always remove the "sent from my iPhone/BlackBerry line in the mobile settings.
It's funny you asked though. My email sig is my first and last name, and then my mobile number. I used to have "BlackBerry: xxx-xxx-xxxx. Since I've had the iPhone I just have "Mobile Number." I guess because BlackBerry used to be more business oriented.
I like seeing when people are sending from their device, I just get a kick out of it. Personally, like was mentioned above, I don't really want to let people know if I'm in or put of the office at the time.