Effie Law and I had a paper accepted to CHI 2007 entitled "Meta-Analysis of Correlations Among Usability Measures".
The paper is a meta-analysis of 73 studies that - via the biggest data analysis I have ever done - leads to a couple of insights about the relation between aspects of usability. There is more information in the paper's abstract; the full paper arrives after the conference at the end of April. If you are impatient feel free to contact me before then.