Thought I would post this as a heads-up.

Bug 46009 - iCal4: Monthly xth day of week recurrence results in weekly recurrence

Apple - Support - Discussions - 10.6.3 iCal custom repeat meetings ...

I'm going to try and see if I can revert to an older version of iCal under 10.6.3, instead of forgoing the OS upgrade. If that does any good, I'll post again here.

This same bug is in Lightning, too, but it's been around for a while: