It seems to be installing a completely new copy of WoW into a subdirectory, the installer even asked me where to put the PTR client. Usually they just get on with it and make a "Wowtest" directory. And it is taking ages. But I'm sure once this is installed there won't be any more patches for me to pick up and I can jump right in and check out the new stuff.
Edit: hah after logging in I get to download another patch. But here's the interesting thing, the login screen is not the Burning Crusade one, oh no we have the Icy Northrend login screen, complete with flying thingie. Most cool.