Like many others here I have a new site (in it's fourth month) and I managed to get it to rank really well on my chosen keywords pre Austin. I lost my main keyword and a few others in Austin, totally gone, nowhere to be seen and I still have some two word key phrases not affected.
I have been reading the forums here and following all of the advice, lots of links, keywords in anchor text etc. Just quietly plodding my way through emailing lots of webmasters
I've made a few minor changes to the site too to see if it would make any difference (basically a bit of keyword stuffing) but it hasn't so I will go back and unstuff the site.
I read here too about another member having had so many errors in their HTML that this could affect their rankings so I went and found an online HTML validator and checked my site (smiley for a major heart attack goes here!). It was bad! I'm no website builder but I've done my best and managed to get the errors down to only 4 (my page looks a bit funny now but hey ho

).
Anyway, my point.... (finally! - it's because I'm a girl you now, can't help but babbly on

) I used the HTML validator to check the top ranking sites for my keywords and all of their HTML is in a right state (not like mine now she says smugly

)
So. Does is affect rankings? Messy HTML hasn't seemed to have any negative affect on the top ranking sites in my industry.
Thanks too for everyone's input here, it's very helpful to newbies like me
Jane