WebSphere Enterprise Service Bus (WESB) is a new product that implements (IBM's take on) the ESB pattern. There's a general marketing overview (PDF) and a set of PDF documentation. The InfoCenter docs are a part of the WebSphere Business Process Management InfoCenter, which includes docs for WebSphere Process Server (WPS) and WebSphere Integration Developer (WID) as well.
The WESB release coincides with new v6.0.1 releases of WPS and WID. The v6.0 versions of those products were released back in September 2005. The v6.0 numbering can be a bit misleading; rather than these being the sixth major release of these products, this is actually the first (essentially v1.0). The version 6 numbering coincides with the numbering of the current versions of WebSphere Application Server and Rational Application Developer. v6.0.0 is the first release of WPS and WID; v6.0.1 is the first release of WESB.