The ASX/200 index is rebalanced quarterly, not monthly. Inclusion is based in part on market cap but there are other factors such as liquidity as well.
The changes to the ASX/200 were announced on 5/12 and take effect from the 3rd Friday in December, which was last Friday the 19th.
The announcement for the December quarter is here: http://www2.standardandpoors.com/spf/pdf/index/20081205IN_QrtlyRebal.pdf
From what I can see, there is no change to CER. Looking at the S&P constituent list, it has remained in the ASX/200 index.
S&P can perform intra-quarter rebalancing if the situation of a company changes materially. A drop in market cap does not mean automatic exclusion. For companies in the index who are experiencing some distress, S&P give work-out time before removal if there is a chance they will recover.
I think CNP were taken out of the ASX/200 at the end of September. The announcement of this is here:
