First, if you are reviewing every comment before publishing it (I assume in your case so because they are emailed to you) I'd say you've got little to be concerned about.
It does seem like a good idea to create a semantic separation for UGC - however, I don't know of any way to do that, not even in HTML5. I know I get frustrated when an article with a lot of comments comes up in the SERPs for something in the Comments section but not for the articles content. I would at the very least create a <div> with a separate ID attribute.
The only other approach that I can think of is something that some sites have done - create a separate URL just for the comments and display that content below the regular content in an iframe. Seems like a lot of work to me, especially for people who arrive directly at the comment URL, but it might be worthwhile for some situations.