A new version of OpenTTD 0.5 was released a few minutes ago including important bugfixes and an experimental threading support for MorphOS (e.g. allows progress information while generating a new landscape).
The OpenTTD Team released an update for OpenTTD 0.5 branch which fixes many known bugs and adds a few new features. You can download it with BitTorrent directly from http://torrent.openttd.org/ (very fast).