In his post PCs at a Crossroads Michael Friedenberg reports on IDC's measurement of the PC marketplace. From the article:

"Case in point is the PC market. Market researcher IDC reports that 2009 will be the first year since 2001 where PC shipments will decline. I believe this drop is driven by a more rapid intersection of the cyclical and the systemic as the PC value proposition is challenged and then transformed. As Intel CEO Paul Otellini recently said, "We're moving from personal computers to personal computing." That comment signals Intel's way of moving into new markets, but it also acknowledges that the enterprise PC market has arrived at a crossroads."

Cloud computing is one development that is dramatically changing the desktop and server eco-system. The performance of a single server or desktop hasn't kept pace with the computational needs of modern science, engineering, or business. Cloud computing moves away from capital equipment to the ability to procure just the computational output AND at infinite scale for most use cases. Most desktops are idling most of the time, but are too slow to get real work done when you need it. This is pushing the work towards elastic resources that are consumed as you go. If the browser is all you need, then a move towards server consolidation and thin clients is not far behind.
Link to original post