A major annoyance with Apple's iCal is that it won't give you reminders that you have set up if you have rebooted or logged out, and then not launched iCal again. After iCal has been run, it leaves a helper app to give reminders, but this doesn't get launched at login.
The iCal helper app is hidden inside the iCal application itself. To find it, control-click on the iCal application and select Show Package Contents. The app is in Contents -> Resources-> iCal Helper.app.
Now open System Preferences and select Login Items. Just drag the iCal Helper.app from the Finder onto the Login Items list, and you're set. You can't just select the iCal Helper app from the Login Items panel as it is inside another application.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030510001910197