... which btw, you can't delete cause most Microsoft tools are compiled with MAX_PATH = 260 chars - Microsoft KB 177665. What to do? Tried a few tools, spent a half an hour on the net and luckly I found this post on David Cumps' blog - big thanks to James C-S and Kenny for a clever solution using Robocopy with /MIR (or /PURGE) parameters to basically mirror a blank folder on top of the folder you want to remove - in my case:
ROBOCOPY D:\empty D:\SVN_ROOT\SVN_ROOT /MIR
I believe robocopy is included with Vista and Windows 2008 Server - for XP and 2003 Server, you find it as part of the Windows 2003 Resource Kit Tools.