According to Matt Cutts's SEO video on qualities of a good site video, he said that Google treats the bold and strong tag with the same weight-age. Well, if you are into the finer details of the semantics of markup then it is prefer to use the "strong" tag. On other hand, if you are into minimizing typing time then it is prefer to use the "b" tag.

When this article was first written, there were evidences that "b" tags had a slightly stronger weight. Currently we know that Google weights them the same, but we haven´t got a specific answer from Yahoo or MSN.