After an undisclosed illness forced her to one concert and postpone two more, Janet Jackson was released from a Montreal hospital yesterday. Jackson reportedly became sick while sound-checking her concert in the Canadian city and was rushed to the hospital just before showtime. Jackson's stay at the hospital was brief, lasting just two hours, but it was enough to determine that Jackson had to "recuperate" from her unspecified ailment. Thus, her Rock Witchu tour stops tonight in Boston and tomorrow in Philadelphia have been postponed. Barring any health setbacks, Jackson should be recovered in time for her concert Saturday night in Greensboro, North Carolina. Jackson also canceled a concert last week in Detroit, but that was due to "production restraints."