One of the first things I look into on an alternative OS is how it supports my language. You see Greek is not a standard Latin lettered language. It has it's own alphabet and accenting system. Well, Haiku does that well, although the application to change keymaps on the fly was not included in Alpha 1. You can find it here and it is called KeymapSwitcher with the latest version when this blog post was written being this. Just unzip it in /boot and make a shortcut for /boot/common/bin/KeymapSwitcher on the Desktop Applets folder (perhaps that should have been already in the zip file).
Actually Haiku's input server is better than the original BeOS one, which handled the dead keys very badly when having to do with two byte characters such as Greek. Very well done guys...