Someone asked one of the industry listservs recently if trade shows were on their way out. Some may well be, depending on what industry they serve, and how many shows are already working in that space, and how many alternatives their prospective exhibitors and attendees have to learn about new products and services. Some will lumber on due to legacy momentum; others will nimbly hop to new platforms to keep up with what their attendees and exhibitors need and want and do just fine. Others will find it just business as usual. Yet others will likely grow as they find new audiences and product areas related enough to make it make sense to expand in their directions. That's what I think, anyway.
Here's another view, from Dan O'Shea at the Big Fat Marketing Blog.