Hey hey HEY Hirst! Watch the "primary concern" racket.
Seriously, there are direct and indirect SEO implications. I'm going to interchange semantic markup (which I find to be a CSS trendwhorish, misunderstood, overused term) with quality code (which I find describes things more accurately).
Indirect SEO implications
1) Some webmasters, when considering an inbound link, consider the quality of code. Quality code keeps those people happy. I've even seen directories (I forget which ones) that make quality code a prerequisite for inclusion.
2) Quality code provides faster load times and therefore an enhanced user experience. This, in turn, leads to more of the organic backlinks that we all covet.
The beauty of these links goes far beyond SEO in that these links quite often send something that is apparently a foreign concept these days: direct user traffic.
3) Quality code enhances the user experience and keeps the user around longer. This isn't a known factor...yet. But with every major SE having a toolbar, it would be relatively easy to track user experience by the time spent on a site, pages visited, etc. and incorporate it as a ranking factor (and then have to deal with the manipulation afterward, which is always 10 times harder). I'm a believer that this will be a factor sooner rather than later, if it's not already.
Direct SEO implications
1) Quality code is easier to read and interpret and therefore reduces the risk that a spider will misinterpret something.
2)
http://labs.google.com/accessible (Google's Accessible Search) is something that's been very quietly in "Beta" mode for at least 6-8 months that I've known of (probably longer...I've never bothered to look).
Do a search on a term using google.com and then do a search for the same term using the accessible search. The quantity of results is significantly less (usually 90% less is what I've found), but the quality is generally much higher with very little spam. It's not perfect by any stretch of the imagination (I've seen a few sites with some accessibility issues make it in there, although nothing really egregious), but I've gotten to the point where I use it for regular searches because the results are that clean.
Why is this significant? It gives webmasters who do things properly the opportunity to reach a large market (the disabled market) with relative ease since the competition here is next to nil.
I'm also of the belief that big G will make this a factor at some point because spammers generally are crappy coders, so there would be a certain serendipity at foot there. That's strictly speculative on my part, though.
3) Sites that are built well will find that they pick up a lot of longtail and niche phrases with a lot less difficulty than poorly-coded sites. If a webmaster is really lucky, (s)he will find him/herself with a high-volume 2-3 word phrase that no one else is targeting and that (s)he never would have thought of.
I've got an SEO term with the SEFL site (not gonna reveal it here just because spammers are idiots and I'm not opening up a door for them) that's a classic example. I never even CONSIDERED it at any point as a keyphrase, but I rank #1 across all three engines for it and it sends traffic. (Accidents ROCK, and they make you look and sound so much smarter!)
vangogh, if you're around, I think you know the one I'm talking about. Everyone else, if I trust you I'll PM you the phrase. If I don't...well...you ain't seein' it.
