I find the calendar to be the most inflexible of those regarding BES/Outlook integration. The new SP that's about to come out may make it easier or change thing, but as of yet it still only works with the primary calendar.
We use Sharepoint to maintain many calendars respective to different departments, teams, projects, etc. (I have 6 calendars in my O/L 2007 clients), some of them use Google to bridge the gap, having it sync up calendars then viewing them on their handheld's browser via Google.
Best bet for now, unless someone has a better idea...
Hope this helps, let us know if you have come up with a better solution.
So many policies, so little time :-D
Los Angeles, CA
BES Admin 4.1.5