From the announcement:
GreenButton now enabled for Blender 2.5!
We have had many requests to enable GreenButton for Blender 2.5 and we are now happy to announce development is finished and it is now ready.
Please note that Blender 2.5 itself, is still a BETA release, and therefore changes to the api - including changes that may not be compatible with our plugin - could occur without warning in later releases. This plugin is built to work with the official release of Blender 2.55 and we recommend use of this version until we are able to update the plugin to work with later builds.
We would love to hear any feedback you have or if you have any thoughts you would like to share.
Please contact Cassie at [email protected] with your thoughts and feedback.
The GreenButton Team
GreenButton has also recently released their new subscription model which offers a shared pooled option for US$19.95 a month.