This quite recently published article examines subject coverage in Google Scholar:

Neuhaus, Chris, (2006) Ellen Neuhaus, Alan Asher and Clint Wrede The Depth and Breadth of Google Scholar: An Empirical Study
Portal: Libraries and the Academy Vol. 6, No. 2, pp. 127-141.

From 47 databases 50 article titles were randomly collected from each database and compared with Google Scholar. Look at the figures here:

Conclusions: Each databse of 47 (with 2350 randomly selected articles) had a median and average coverage of 60%.

Neuhaus et al means GS weaknesses in subject coverage are social science and humanities databases and strenghts science and medical databases, open access databases, and single publisher databases.