Interestingly, a successful company/product in a category is probably more likely to discourage competition than several that have failed to gain traction. The reason is obvious, but this is probably backward. A success shows that people want what's being offered -- and there's usually room for a couple different twists on something people really want. While, a category that has yet to find a winner is a very strong sign that some assumptions about people's desires are wrong -- which is not going to be changed much by a new twist or better execution.
(Sent vis Gmail to blog from my treo from the only coffee shop on Valencia without wifi.)