That's what eventful.com is. They spider other event sites (including the likes of meetup.com), allow category and location monitoring and provide a cleanish gui. They even offer a developer API if you want to build something on top of it.

What eventful don't have is a good search or good marketing. But neither of those are insurmountable problems, and I'm sure they'll tackle them in time. Or just get bought by google who can deal with both of them.

However one problem is that many events are not listed on any event directory, but just on individual websites, those are much harder to spider and deal with.