Enter Robert Blum's iCalFix, which I found yesterday while surfing for some completely unrelated topic. iCalFix will automatically put a 24-hours-before alarm on any new iCal events you add to the calendar. That's all it does, as of right now -- in version 0.2, Robert promises you'll be able to change the default 'time before' for the alarm setting.
I've been using this for a couple of days now, and it hasn't caused any issues as of yet. However, as with anything that modifies an application, it's a good idea to make sure you've got a current backup -- in this case, of your iCal data files -- just in case something does go wrong.
Note that you'll need Mike Solomon's SIMBL Input Manager (Mike's also the author of PithHelmet for Safari). Some people may have an issue with using SIMBL, as it allows modification of a program's code. I've been using SIMBL for quite a while now (it's necessary for many other cool hacks, such as AcidSearch), and have yet to have any issues I could trace back to it, so I feel relatively comfortable having it installed.

