tedster - 7:16 pm on Jul 13, 2011 (gmt 0)
I personal think everyone should get behind valid mark-up and learn all the goodies that HTML5 and microdata have to offer. But search engines cannot depend on web authors to be all that good at HTML. If they required that, they would miss a lot of the web's top content.
So we might get a small edge using 100% academically correct mark-up, and our page' specifics may certainly be clearer and easier for Google to deal with, but it's not going to be major advantage, it's an incremental "add-on".
As pageoneresults said "you may be able to nudge your way up a spot or two".