I like to consult the Webster before any other source when it comes to definitions. I agree that we're being sidetracked by semantics, but as long as that's the case, might as well be as accurate as possible. I've bolded that portion that I think is most important. I don't actually agree with the second definition of efficient... efficient to me isn't about being effective, but anyway:
Efficient
Adjective
1. Being effective without wasting time or effort or expense; "an efficient production manager"; "efficient engines save gas".
2. Able to accomplish a purpose; functioning effectively; "people who will do nothing unless they get something out of it for themselves are often highly effective persons..."-G.B.Shaw; "effective personnel"; "an efficient secretary"; "the efficient cause of the revolution".
Source: WordNet 1.7.1 Copyright © 2001 by Princeton University. All rights reserved.