[drupal-devel] Theme engine roadmap

I'm working on an original theme that I hope to contribute for inclusion in Drupal 4.6 as a default theme (assuming it is finished in time). Of course my dedication might wane before then as well... Can anyone in the know clarify the future plans for core theme engines. Based on comments at http://drupal.org/node/6493 and elsewhere XTemplate sounds doomed but phpTemplate has not been included in the distrubution to this date so it clearly is not an option. More importantly, what are the performance and code considerations for implementing each engine as the default? If this is documented somewhere else just point the way. Cheers, Chris