An example Gradle build configuration that shows that `ext` does not always point to the same object as `project.ext`. The assertion on line 10 fails.

build.gradle

assert(project.ext == ext)

// assigning some values just for testing
project.ext.set('a', 'a-val')
ext.set('b', 'b-val')

assert(project.ext == ext)

task test() {
    assert(project.ext == ext)
    doLast {
        assert(project.ext == ext)
    }
}

assert(project.ext == ext)

Here's the explanation:
http://forums.gradle.org/gradle/topics/project_ext_and_ext_differ#reply_11889305