Regarding half-screen on Linux: it's still Alt+Enter, but I found that sometimes you need to have the console open to make it work ie. when console is open it will always work, with console not open it will work only sometimes.
Some other stuff to watch out for:
1) If you use Compiz or have window manager compositing enabled, don't. Disable them if you want halfscreen. When trem is running, its gamma affects everything if those are enabled. Basically you get a very bright and white desktop if you halfscreen.
The same applies to the resolution: when you halfscreen, if the trem resolution is not the desktop resolution, you'll see the desktop at the trem resolution.
2) If you're on another workspace editing and trem loads a new map, it will fullscreen over the current workspace. But it only takes over the display, the keys and mouse are still doing desktop stuff, not trem stuff. To escape this silly situation, use whatever keyboard shortcuts you have for switching back to the workspace trem is on (if you don't have any defined... tough; go get some -- I use Alt+digit for this). Once you switch to that workspace keys and mouse go to trem again and you can halfscreen back out of it.
PS: the above applies to Gnome/Metacity/Compiz environments. No idea if it affects Emerald, KDE, XFCE etc. the same way but I'd venture to guess it does.
