The problem with signature based virus scanners is that they take a reactive approach to security. Only after a new virus is discovered to signatures become available, so there is always a window of vulnerability. As virii become more sophisticated and propagate faster, this lag becomes more of an issue.
A heuristic virus scanner is one that recognizes virus like behavior rather than depending on signatures. It will be interesting to see if more companies start to take this approach to virus detection in the future.