Good clickz article covering the fact that advertisers and publishers have found a model for the advertising on internet that actually works. Publishers of all sizes are creating content to sell ads - and they're focusing on areas that have higher value. Inventory (i.e. web content that sells something) is purposefully being created:
In other words, it's a marketplace responding to demand. Sounds great, right? Well, it is great and it is a better model than the dot-com bust days. The market is actively searching out what sells and creating more of it.
The only downside is the eventual regression to the mean. Right now, it's easy money for all involved. Ad networks, publishers, search engines, agencies, SEO and SEMs are all doing well. So well that they are attracting more investment, more people, and more capacity into the market. This in turn creates more supply.
The internet scales easily and more supply can engender more demand to a point. However, eventually ROI will decrease as more money and people chase the same goals. It's a classic boom time. The hard part is predicting how far into the boom, and how close to the bust we really are.