The i vs em and b vs strong debate is as heated as the adsense debate is here. i and b (supposedly) rely on the fact that there is an actual corresponding italic or bold font on the viewer's computer for whatever font is in use. For example if Helvetica is the font in use, helvetica italic would have to be installed on the computer or specifying "i" would have no effect). Strong and em on the other hand do not rely on there being a corresponding bold or italic font on the computer.
I was actually going to do a test HTML page and test all of this out, but lost interest. There is a possibility that modern browsers will compensate for this and it doesn't really matter which one you use, but I've never found any indication that browsers will substitute em for i if there is not an italic font installed on the viewer's computer.
In the real world, does it make any difference? I don't know for sure, but I do know that when using em and strong, there is less likelihood of running into problems.