I have iCal synced with a google calendar account. I only have one calendar. When I disable an alarm for an event, a few seconds later the alarm keeps turning back on. This happens for all of the events I've already added. I've tried re-adding the account in iCal, same thing happens. I'm thinking it keeps syncing the alarm from google calendar and never pushes the updates to google calendar.
Same thing seems to have started happening to me as well, recently. It's on both my Leopard and Snow Leopard macs.
If I turn the alarms off either on my iPhone (push synced to Google via Exchange) or the google website they go away in iCal as well.