I guess most of us having My Oracle Support have got email from Oracle mentioning that Critical Patch Update January 2012 has been released. Main reason behind this is Security bug found by Infoworld which can make database run out of SCN. You can read the article here
Summary
There is bug 12371955 which causes scn to be incremented at very high rate when we issue begin backup command in 11g database. There is already patch available for this.
But Infoworld seems to have found security flaws which can cause scn to be be raised by say 1 trillion on interconnected database. Jan 2012 CPU patch seems to address this issue.