I just installed OSX 10.6.8 on my iMac and when I went to the Minecraft launcher and clicked play the launcher would go away for a few seconds (like it usually does before loading Minecraft) but then it would just bring me back to the launcher instead of going to the title screen of the game.
Please attach the complete output of the "Development console" tab of the launcher.
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If the launcher closes after game start, please edit your profile and select "Launcher visibility" then, "Keep the launcher open".