Is a cover letter required when enquiring about the potential to do a post doc in a lab when no position is advertised?
In my first post-doc currently - I sent a couple of solicitations out to some labs while applying to posted positions (one of which I got). Remember this - professors receive a dizzying amount of email in a given day; including a cover letter with your career goals, research interests, and a description of how those fit with the work of the lab you are seeking to join would likely go a long way in getting his/her attention. You don't want to end up in the numerous emails that go straight to the trash - show this potential employer how serious you are!