There are other major factors preventing the rise of the Linux desktop. First and foremost is business computing. While it's certainly possible to set up and configure Linux desktops with centralized LDAP, it's not as easy as Microsoft's Active Directory. In fact, if there's one thing that Microsoft has done extremely well, it's AD. That, coupled with the Office suite and Exchange, makes the case for Windows in the workplace. There will be Macs and perhaps even Linux systems on the same corporate network, but they'll be one-offs and most likely will be configured to play nice with Active Directory. Thus, Microsoft owns the corporate network.
To change this, a few things need to happen. Obviously, a realistic substitute for Active Directory and Exchange has to be developed and be significantly cheaper than Microsoft's offering. There are plenty of ways to handle e-mail, and OpenLDAP and others can certainly perform directory duties, but there's no clear-cut management interface that ties them together in the same way that AD and Exchange are managed. That's absolutely key.
Imagine that for a few hundred dollars, a medium-size business could implement a fully AD-compliant directory with e-mail support identical to Exchange, with support for Windows, Mac, and Linux clients. Imagine that there was no licensing to worry about, and a client desktop can run on any hardware and run Windows applications via virtualization or WINE. My guess is that it would be a big hit. Apple could do something like this if it put its focus on business computing, and any number of Linux vendors and ISVs have tried to do this in the past with mixed results.
However, times are different now. Microsoft's waning along with the economy, and businesses are more cost-conscious than ever. Vista was released too late, requires significant hardware upgrades, costs too much, and is generally viewed as a failure. The time is right for a real competitor of Microsoft to appear in the business network.