Yes, but they can't use it to take pictures while in the polling place.
Poll Watchers should also make an attempt to keep any noise (from calls or ringing) to a minimum so as not to disrupt voting.
For additional information, review the Code and the regulation adopted by the State Board of Elections.
D. Authorized representatives shall be allowed, whether in a regular polling place or central absentee voter precinct, to use a handheld wireless communications device but shall not be allowed to use such a device to capture a digital image inside the polling place or central absentee voter precinct. The officers of election may prohibit the use of cellular telephones or other handheld wireless communications devices if such use will result in a violation of subsection A or C of § 24.2-604 or § 24.2-607.