I got problem, when i clicked page tree it always loading and nothing changed.
When i checked with FireBug, i found request problem like this:
http://www.mydomain.com/sapphire/third?_=1291343199611 --> 404 not found
Later i found this problem only happened if i login with www: http://www.mydomain.com/admin
But if i login without www: http://mydomain.com/admin, everything is ok.
I had try to delete all silverstripe files and re-install for about 6 times with different version of silverstripe but the problem still happened.