Importing or sharing calendars is very handy when 
creating many projects across a department.

That way a single change to the common calendar
e.g. adding a public holiday/vacation day, 
allows many projects to automatically inherit 
this change when they are opened for review.

Given that Planner uses an XML file format it 
would be relatively easy to make a calendar just a hyperlink 
reference to another planner file. This common
file could simply be an empty project (no tasks
or resources etc) just like a sort of master
project file.

Actually it could also store the common resource names 
too but I'll raise a diffrent bugzilla for that.

That file could either be remote (http://) or local 
(file:/// with regular URI conventions (for GNOME). 

The updating is probably easy to set as an on-demand 
refresh of links - maybe some stale link checks (per-day),
with local caching.

The planner simply gets the remote document, parses 
it and picks out the relevant calendar (or calendars) and 
then sets itself up with those details.

A further enhancement would be to hook into ical/ics/idt 
remote calendar servers (e.g.
http://www.brownbearsw.com/ical/idts/swed_hol.idt