I had given up on MS (after buying it, but bailed because they don't answer tech support email questions) and went to PM. Then Leopard broke it. bummer.
I gave MS another try and, deciding to be bold, actually did what they recommended in the tech support section, and guess what, it works! Amazing. Maybe I'll start asking for driving directions when lost. Maybe.
Plagiarized text follows from the MarkSpace Web site (sorry if that breaks rules, but I couldn't get a direct link.) Follow all the steps, don't try to get tricky and skip one, it won't work.
To see this live, go to https://support.markspace.com/index.php
and do the troubleshooting wizard.
1. Go to iCal and click the 'File' menu and choose 'Back up database'.
(Leopard - 'File' menu and choose 'Back up iCal')
2. Now revert to the same database you just backed up, click on the 'File' menu and choose 'Revert to database backup'.
(Leopard - click on the 'File' menu and choose 'Restore iCal')
3. Do the same process for Address Book click the 'File' menu and choose 'Back up Address Book'
(Leopard - 'File' menu and choose 'Export - Address Book Archive')
4. Now revert to the same database you just backed up, click on the 'File' menu and choose 'Revert to Address Book backup'.
(Leopard - click on the 'Import' menu and choose 'Address Book Archive')
Steps 1 through 4 are key to the process, they are not just a backup, but the revert part forces a reset of the relationship between Sync Services and iCal and Address Book.
5. Launch the iSync application, found in the /Applications/ folder. (iSync is not used for the sync, but iSync is where Apple handles Sync Services Preferences.)
6. Click on the iSync menu
7. Choose Preferences
8. Click the 'Reset Sync History' button. Read and follow the onscreen directions.
9. This may launch iCal. When the reset process has begun the button will go grey, it is complete when the button becomes active again, but there is no need to re-click it.
10. Restart the Mac
11. Soft reset your BlackBerry. Turn it off, pull the battery for 3 minutes, put it back and turn on.
12. In Missing Sync for BlackBerry, double click the Calendar plugin and reselect the calendar for the field: "Put events created on the device into:"
13. In Missing Sync for BlackBerry, double click the Tasks plugin and reselect the calendar for the field: "Put tasks created on the device into:"
Then, try the sync again.