Are annual cost of living raises in Social Security guaranteed?

Since 1983 annual cost of living raises for Social Security benefits have been paid based on the average % increase in the CPI-W (consumer price index for urban workers) in the third quarter over the average CPI-W in the 3rd quarter of the previous year. The COLA paid in January 2009 was 5.8% and the average over the past 20 years has been 3% per year.

The automatic COLA provisions provide retirees an important protection against the erosion of their standard of living due to inflation. For example, if the average COLA continues to be about 3% for the next 20 years, a monthly benefit of $1000 in 2009 would be increased to about $1806 in 2029.

But as we all know, these are not normal economic times. What happens if the CPI-W continues to decline as it as done since the 3rd quarter of 2008?

If that happens, Social Security benefits would not be decreased, but there would be no automatic COLA raise in January 2010.