I think this is a Spotlight thing: Spotlight is only able to find
files. So, if you want to be able to search for iCal events using Spotlight, they need to each occur in their own file somewhere. So, as of Tiger, I think iCal has a folder with a whole bunch of individual iCal events or ToDo items, each in its own file. Spotlight then indexes these files. (It's smart enough to parse them, etc.) (I think, as Ferd writes, you can find .ics versions in ~/Library/Application Support/iCal/Sources/ in files called corestorage.ics.)
However, iCal still uses .ics files for importing and exporting, etc.
By the way, if you want to find these files, just do a search for ".icalevent" or ".icaltodo" (the extensions on these files) and all of these files will show up in the search results.