What can you do if a meeting participant is constantly on their cell phone, blackberry?
This is a common occurrence at all meetings. Start with some upstream prevention at the beginning of a meeting to outline ground rules with the group. Discuss, what the group wants to handle the use of technology during their meeting time. Ideally, people will agree to mute their phones and respond to messages either during the break, or outside the meeting room. Some groups even go as far as to impose a "consequence" for not following the ground rule - $20 to charity may be a good incentive.
If during the course of your meeting, people are using their phones, as the facilitator, you should intervene. You can either move towards the person and with your physical presence the person may stop, if not, you can quietly request that s/he check messages outside the room.
It's important to remember, that even though responding to e-mails on a phone/black berry is quiet, the person on the black berry disengages from the meeting and can be disrupting to others. It is worth addressing this as soon as it occurs so that it doesn't drag on through the full meeting.