I've only noticed this behaviour once in Mountain Lion 10.8.2, but Sublime Text 2.0.1 (Build 2217) suddenly started ignoring the setting
"open_files_in_new_window" = true
Restarting the editor did not solve the problem.
The behaviour was fixed by setting this option to false, then back to true again.
I was not doing anything special at the time, so don't really know how to reproduce the bug. Hope this helps someone!
Customer support service by UserEcho