There is a nice summary of articles and resources on the nature and disadvantages of spec work in the design field on David Airey’s site. I wholeheartedly agree with Debbie Millman (President of AIGA) — the focus of the article — that the practice is detrimental to both clients and designers.

During the summer before my senior year in college, I agreed to do a half dozen mockups as part of an interview process for a small design firm in Rochester. I never heard back from them, and a few months later saw a remarkably similar design to mine in use on a billboard for the client they represented. The prospect of tangible work can be alluring to design students — in limited markets such as upstate New York it can very difficult to find jobs or internships — but the disadvantages will always outweigh the potential.

AIGA President Debbie Millman on spec work, David Airey
Interviewing Debbie Millman: President of the National AIGA, No! Spec