I installed a fresh copy of Silverstripe right off the trunk and was able to login into the administration interface. After including Subsites and rebuilding via /dev/build the admin interface stopped working with:
Fatal error: Call to a member function getVar() on a non-object in /srv/www/silverstripe.local/silverstripe-trunk/cms/code/LeftAndMain.php on line 1005
I fixed the issue with the patch I attached, but since it worked without the Subsites module I wonder if the cms-code is the right place to fix this.
This is irrelevant now :) closing.