My motherboard is an ASUS P6X58-E WS.
Note that this CPU and motherboard combination worked well with linux-3.1 without any warning.
Only a software upgrade to linux-3.2 caused this error message.
Further checking on the changelog for the debian package intel-microcode shows that rev-0x13 microde for cpu 0x206c2 was released on Sept. 2010 and withdrawn 2 months later on Nov. 2010.
It's unclear why the newer linux image would be requesting a microcode version that was withdrawn a year earlier while the previous linux version, released in Jan 2011, did not.