There won't be a party. Just a press release. I'm not sure if they would do it on the exact same day or not.
I think it depends on what is upgraded. If it's just an Ivy Bridge w/USB 3 (i.e like the MBA and non-retina MBP), then I agree with no event. However, if it's a more substantial upgrade (i.e. Thinner redesign, retina, etc.), then I think there would be some sort of event.