What do you mean by out-of-the-box portlet support? As in tapestry-core itself? I'm sorry, that's not going to happen. Not everybody which uses Tapestry uses portlets, so it doesn't make much sense to put support for it in core. The Tapestry philosophy is to have the basic functionality in tapestry-core and more specific stuff in other JARs, such as tapestry-upload and tapestry-hibernate.
If it's about a separate JAR under the Apache Tapestry project, tapestry5-portlet is mostly written by Tapestry committers themselves. In addition, we avoid having too much stuff under the Tapestry project itself because we're all volutaries and we'd need to support that code.
I agree with you that tapestry5-porlet should have its JAR in Maven Central Repository.