But I've been able to transfer GroupWise calendar info to iCal and my iPhone, (neither supported by GroupWise or my employer) via a BlackBerry which gets data from a BlackBerry Enterprise Server. Why bother if I already have a BlackBerry? Get an iPhone and you'll find out...

Instructions for doing this are in the Mark/Space knowledgebase, but fragmented and hard to find. Basically,

1. create a new calendar in iCal,
2. tell MissingSync that it's the one you want to use,
3. disable wireless synchronization for the blackberry calendar, connect via USB and synchronize.
4. Disconnect and re-enable the blackberry's wireless sync.

Repeat steps 3 and 4 as often as needed to keep iCal up to date. Sounds like a pain but it's pretty simple. The only catch is that some recurring GroupWise events transfer erratically. This is due to the ridiculous way GroupWise creates recurring events.