If you measure interest in operating systems based on search, Linux hit its peak in 2004 and has been on a downward slide ever since, according to Google Trends. The same is true of Windows, though the trend (as seen below) is less pronounced. The Mac? Well, let's just say that people can't seem to get enough.
But what happens if we measure interest based on press mentions? While all three operating systems seem to have increased their "mindshare" over the past four years, Windows has outpaced Linux and Mac OS, and by quite a margin:
What this does not measure, of course, is whether this interest is positive or negative. Much of Windows' news coverage has centered on Vista delays and its underwhelming of expectations. Not exactly what Microsoft wants people saying about it.
Regardless, these are imperfect measures of importance. The standard bearers for each OS - Red Hat (Linux), Apple (Mac), and Microsoft (Windows) - continue to blow out their numbers. Apparently any interest is good interest, at least as far as sales go.