Overview

This has been known to occur in Eclipse in both 3.1 and 3.2 using Mule IDE 1.3.0 when setting a version of Mule other than 1.3.3 as the default distribution.
That is, if in Window->Preferences->Mule you add a Mule installation other than 1.3.3 and either it becomes the default (checked) or you set it as the default, then this issue will arise.

Resolution

You can 'workaround' this issue by leaving mule-1.3.3 as the default, but when creating a project setting a non-default installation as the Mule version used. You may then need to manually import the version specific mule libraries (using a user library) and remove the mule-1.3.3 distribution from your build path.

Warning: This workaround may affect the transportability of the eclipse project.