Based on the difficulty level of paper 1 and assuming an average attempt of 80 questions out of 100, a minimum score of 90 marks out of 200 in paper 1 after deducting 1/3rd negative penalty. In paper 2 assuming an average attempt of 70 questions out of 80, a minimum score of 130 marks out of 200.
Combining the 2 papers the Cut-off score for Prelims 2012 comes to 90+130=220 marks out of 400. In fact, It can be predicted that the cut-off marks will be in the range of 215-230 marks simply because of the easy nature of paper 2 and not-very difficult level of paper 1.
Disclaimer: This cut-off is for reference purpose only, and this is not official released cutoffs by UPSC