I am asking the same question.
In Outlook 2003 the 'View Group Schedule' feature shows calendars laid out
horizontally, like a wallplanner which is very useful (like the 'Attendee
Availability' page in the 'Appointment' form).
The infoselect Group Calendar (linked from the great Slipstick website
http://www.slipstick.com/calendar/scheduleall.htm) is a client add-on that
seems to give more wallplanner scheduling features (although I haven't
tried it yet).
Also, RBSchedule at http://www.rbware.net/rbschedule/ looked interesting.

If anyone else has any ideas for wallplanner style calendar displays,
please share!