The DailyMail outlines what Hawking's party was about:
It is the ultimate test for time travellers – hold a party for them, but don't post the invites until after the event.
But sadly, when Stephen Hawking tried this, he was left to his own devices – for across all of time and space, his invite to literally anyone who ever will exist landed on deaf ears.
Professor Hawking offered this as 'experimental evidence that time travel is not possible' – although we secretly hope that by publicising the party's existence (held on June 28th 2009), someone in the future might retroactively decide to visit.
The first time I heard of someone doing this was a confernce at MIT in 2005. They figured you would only have to have one time travel conference ever because everyone in the future could get there. That of course means time travelers from the past wouldn't know about it. I guess they just figured that nobody has invented a way to successfully time travel yet.
Here's their summary of what happened at their conference:
The convention was a mixed success. Unfortunately, we had no confirmed time travelers visit us, yet many time travelers could have attended incognito to avoid endless questions about the future. We had a great series of lectures, awesome bands, and even a DeLorean. We regret having had to turn away visitors, but there were capacity restrictions governing Morss Hall. Thanks so much to the dozens of people who helped.
Image from Tao Zero on Flickr