1) A comparative phrase meaning "possessing no great dissimilarity from, but not neccesarily possessing similarity to."
That subtle denotation escapes most, and it is most commonly used as:
2) a clumsy, noncomittal way of expressing simple similarity.
1) Well, yes, I guess it would be fair to say that Bush's appropriation of power for the executive is not unlike the administration of Hitler's Germany, but that misses the point.
2) He looks not unlike Elijah Wood.