Anchor text in the SEO world is part of the large determinant of inbound links. In recent years, many SEO campaigns have created high volumes of keyword anchor text links in an attempt to show relevancy.
Post Vince update (mid 2009), webmasters have to consider how to demonstrate to search engines that they are an online brand. Gradually in the last 12-18 months, reputable online PR and SEO agencies have realised that a website's link footprint is an online illustration of its brand values. So image links, domain links, 'click here' type anchor text links and even nofollow links are all indicative of a classic natural link footprint. I've worked on a couple of projects where we've needed to address to the volume of keyword anchor text links - this largely down to inheriting bad SEO campaigns that have placed poor quality site wide links in an attempt to spam SERPs.
So I have 2 bits advice to anyone new to SEO looking to improve a site's rankings and search visibility.
1) If you are working on a new web initiative, be extremely careful when picking your domain. If you select a domain with one or all of your core relevant keyword terms then you give yourself a competitive advantage straight away. If however you inherit a domain like brandname.com without any keywords and it is a well established domain that you/your client wants to keep then adapt your link building strategy accordingly. Basically if you then create a big keyword anchor text link profile this will blatantly not be natural in Google's eyes.
2) Think about the bigger picture. Examine your link profile objectively and if it looks unnatural to you then in the future at any point you have an unquantifiable risk on your hands - you can address this by (a) taking links down and/or (b) redcing the proportional percentage of keyword led anchor text links.