iCal returns standard AppleScript "date" objects when you ask for event dates and such, so you can dissect them pretty easily. Perhaps the easiest would be to compare short date string of theDateReturnedFromiCal with short date string of (current date).
The standard date object has a whole bunch of other properties, too: month, day, weekday, hour, minute, etc. Unfortunately, these aren't in an easy-to-research location like application's dictionaries are, since they're built into the AS runtime... look for AppleScript documentation on Apple's website for the full list.