Product detail pages tend to naturally have good content for search engines to index because they have lots of text in the form of product descriptions. The challenge is having the same quality content on all pages of the site, specifically category pages and home pages.
Most homepages have a lot of graphics and buttons, not much in the way of text. BrooksRunning.com puts a block of links with specific anchor text below the main section of the site. It doesn’t look to spammy and might actually help visitors navigate to what they are looking for, but mostly it’s just spider food. RedEnvelope.com puts a huge amount of text under their footer along with a bunch of links with anchor text though out. It’s a little bit of an eye-sore and obviously not intended to be read by humans, but it probably works.
Category pages are also a challenge to include text on. Zappos.com throws a big block of text on each of its category pages with links throughout. I can imagine people reading this but mostly its for SEO.
I think these three sites have a good strategy to include more text on their pages but recently Google announced that sites that don’t have much content “above-the-fold” can be affected by a recent algorithmic change. Its speaking more specifically to sites that have a lot of ads before the content, but it will be interesting to see if this strategy of putting lots of copy below the fold has less of an effect for ecommerce sites going forward.