C.F. asks in her blog: Is the Cover Letter Dead?
As far as I can tell from my 10 years of experience of job hunting across 3 countries and 2 continents cover letters are history.
In the good old days of the glossy-paper Resume they might have had an effect.
Nowadays the cover letter has been replaced by a line or two I send when I respond to a recruiter's solicitation. Besides it is rare that companies advertise jobs on job boards by themselves so writing a "love letter" to a recruiter that will want to grill you over the phone anyhow is pointless.
Cover letters might have worked with cold calls but a) I am not good at it and b) they are tossed in the trash anyway.