In TG2 we haven't yet been able to completely separate the GUI thread from all other heavy computation, but we have separated the main bulk of rendering into separate threads. This helps keep the GUI fast when it's in the middle of a render or rendering a preview pass, but there are still parts of the renderer and other CPU-heavy stuff executing in the main thread (with the GUI) so it's not ideal yet. We'll continue to work on that after 2.0 release.
The main improvement that you'll see with multi-threaded TG2 is that rendering completes in a much shorter time if you have multiple CPUs or multiple cores. That also applies to the 3D Preview because it uses the same renderer. I concentrated on the main bulk of the renderer - everything that happens while a single bucket (tile) is rendering - and moved that into separate threads. Now multiple buckets can render in parallel, so if there are multiple processors/cores the render will complete much sooner. Initialisation and cleanup at the start and end of a render still happens in the main thread, as do other things like population calculation, heightfield operators, etc. so there's no improvement there. There's much to work on in the future.
Matt
