WebSockets aren't intended to replace AJAX and even a replacement for Comet/long-poll is not strictly on the cards although there are many cases where this makes sense.
The goal of WebSockets is to provide a bi-directional, low-latency, long-running, and full-duplex connection between a server and browser. WebSockets open new application domains to browser applications where previously it was not possible unless AJAX was used. Examples of new applications can be dynamic media streams, interactive games, or bridging to existing network protocols.
However, there are similar goals between WebSockets and AJAX/Comet. For example, when the browser wants to receive events by the server, surely Comet techniques and WebSockets both should be considered. If your application needs low-latency push events, then this would be a factor in favor of WebSockets. Otherwise...