While it doesn't seem like it should matter what you call the people who come to your meetings, it really does reflect your mindset. As Seth Godin points out, customers (or attendees) are those who buy what you're already selling. Clients (or participants) are those who come to you with an educational/networking/whatever need that you then design the event to fulfill.
As Godin says, "The key distinction is who goes first, who gets to decide when it's done." How many meeting planners really dig in and do the research to find out what people need—or have people come to them and say, "hey, we really need this"?
Sure, everyone does the best they can with evaluation results, surveys, focus groups, calls for presentations, and the like, because that's often all the tools you have to try to get inside people's minds. But wouldn't it be amazing if, instead of putting something together and then asking people to buy into it, people felt free to ask you to design something they desperately want and need? How differently would you approach that conference's planning?