I've been hearing this phrase a lot (it now comes with its own music video), but a picture is worth a thousand words:
This particular picture comes from a discussion of the whole "decline" issue at ClimateAudit.com. The black line is the measured width of tree rings. The red line is the reported global temperature. The "decline" begins around 1960. As you can see, it shows a big difference between what the tree rings say and the weather experts report.
I can't explain WHY there is such a difference... but it sure reduces my confidence in ancient temperature reconstructions. If we can't match global temperatures to tree rings in the last fifty years (when we were actually able to check the correlations), why should we believe anything anybody says about temperatures hundreds of years ago?