Not sub-events, but you can create a folder for, say, "Vacations" and then albums that go under that -- "Summer 2007", "San Francisco", etc. Make the albums smart albums, and your work is done for you!
You can create an album (or smart album) by simply right clicking (or CMD-clicking) on the event and the dialog box will ask if you want to create an album from the selection. Say you do, and the entire event will be an album.