iStockphoto is doesn't get much attention. It is rarely discussed on the big Web 2.0 blogs. It somehow is never included with other 'hot' companies like Digg, Twitter, Pandora, Wordpress, etc... despite having a large (and growing) userbase and a clear business model. And like the other companies I just mentioned - iStockphoto shares another common element: the content is amazing. I believe that digital media is predicated on the quality of its content. With great content not available elsewhere, you can build a business. It works for Wordpress. Twitter will figure it out. iStockphoto has figured it out.
Anyhow, this was sparked because iStockphoto has begun advertising... everywhere. And this isn't remnant inventory (or you'd see their banners on my blog!). My guess is that iStock is starting to test the waters with conversion-based advertising. Run across the internet for a couple weeks and garner enough impressions to determine whether it leads to conversions.