http://london-seo.com/bold-vs-strong-keyword-weight/15/ A little "experiement" i did... (please don't say the keyword/post it on your site, so the search engine results are unchanged )
In the videos, i think on http://www.mattcutts.com/blog/more-seo-answers-on-video/ (havn't re-watched) he claims bold is slightly more weighty than strong. The comment on http://www.mattcutts.com/blog/more-seo-answers-on-video/#comment-60927 also backs this up (that he said this). This is proof.