I blogged about this before as the chicken and egg problem with desktop Linux. There are not enough people using Linux on the desktop for software vendors to port desktop apps to Linux, and there are not enough desktop applications for many people to make the switch to desktop Linux.
In the article linked below, Rosenberg suggests building on the momentum of Firefox to help drive Linux on the desktop, and I suspect that we will not see more than a small increase in Linux on the Desktop use in 2006. However, 2006 could be the year of the open source applications. The success of Firefox could lead users to begin to adopt other open source applications (OpenOffice.org, Thunderbird, etc.) As people become more comfortable with open source applications, this trend could drive a few more people to Linux on the desktop. However, we need to get over the chicken and the egg application dilemma before we will see broad adoption of Linux on the desktop for sophisticated business users with large numbers of applications.