HTTP 400 (bad request) for logical error, not malformed request syntax

As of this time, the latest draft of the HTTPbis specification, which is intended to replace and make RFC 2616 obsolete, states:

The 400 (Bad Request) status code indicates that the server cannot or will not process the request because the received syntax is invalid, nonsensical, or exceeds some limitation on what the server is willing to process.

This definition, while of course still subject to change, ratifies the widely used practice of responding to logical errors with a 400.

Status 422 (RFC 4918, Section 11.2) comes to mind:

The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions.