Assuming your publication is named `main`, the following should work. You may have to adapt `groupId` and `artifactId` as needed.

jar {
into("META-INF/maven/$project.group/$project.name") {
from generatePomFileForMainPublication
rename ".*", "pom.xml"
}
}

I thought it was something like that, but I get this (the publication is mavenJava):

• What went wrong:
  A problem occurred evaluating project ':example'.
  > No such property: generatePomFileForMavenJavaPublication for class: org.gradle.api.internal.file.copy.CopySpecImpl_Decorated

This might be related to the new configuration approach used for the `maven-publish` plugin. I recommend to try with the latest nightly build.

Ok, I will give that a shot when I come in tomorrow. Thanks for the replies!

Still doesn't work with the nightly.

$ gr :example:clean :example:jar
Parallel execution with configuration on demand is an incubating feature.

FAILURE: Build failed with an exception.

• Where:
  Build file 'C:\path\to\example\build.gradle' line: 3

• What went wrong:
  A problem occurred evaluating project ':example'.
  > No such property: generatePomFileForMavenJavaPublication for class: org.gradle.api.internal.file.copy.DefaultCopySpec_Decorated

• Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 5.898 secs

$ gr --version

------------------------------------------------------------
Gradle 1.9-20131003220019+0000
------------------------------------------------------------

Build time: 2013-10-03 22:00:19 UTC
Build number: none
Revision: 7970ec3503b4f5767ee1c1c69f8b4186c4763e3d

Groovy: 1.8.6
Ant: Apache Ant(TM) version 1.9.2 compiled on July 8 2013
Ivy: 2.2.0
JVM: 1.7.0_21 (Oracle Corporation 23.21-b01)
OS: Windows 7 6.1 amd64

Here is the jist of my build script (The line number will not match with above because this is actually a multiproject build with some of this in 'subprojects sections of the main build.gradle)

apply plugin: 'maven-publish'

publishing {
publications {
mavenJava(MavenPublication) {
from components.java
}
}
repositories {
maven {
credentials {
username snapshot_user
password snapshot_password
}
url nexus_snapshot_url
}
}
}

jar {
into("META-INF/maven/$project.group/$project.name") {
from generatePomFileForMavenJavaPublication
rename ".*", "pom.xml"
}
}

dependencies {
testCompile "junit:junit:4.0"
}

Have you verified that you can run `gradle generatePomFileForMavenJavaPublication`?

Yes, and it generates the pom I would expect. I did notice this in the documentation:

>The “maven-publish” plugin leverages some experimental support for late plugin configuration, and any GenerateMavenPom tasks will not be constructed until the publishing extension is configured. The simplest way to ensure that the publishing plugin is configured when you attempt to access the GenerateMavenPom task is to place the access inside a publishing block, as the above example demonstrates.

Do I have to wrap this jar part in a publishing block? Would that even work?

I guess it would.

I tried wrapping the jar part in a publishing block, did not help, same error.

Is there a work-around that can add "build/publications/mavenJav/pom-default.xml" to "META-INF/maven/$project.group/$project.name" as pom.xml? What about the pom.properties file?

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Judging from the comments the user found a "workaround" for the issue.

It doesn't look like there any workaround in comments for this issue.
This issue is still actual. Well, at least for my project.

Further discussion here: https://github.com/gradle/gradle/issues/1278