The most recent IDC worldwide quarterly server data says 2008 Microsoft (MSFT) Windows-based server revenue was flat vs the second calendar quarter of 2007. This follows and is no doubt related to the burst in the same category in quarter 1 2008.
The chart shows revenue in billions for each quarter for Windows-based servers as well as Linux-, Unix- and other-operating-system-based server shipments from the factory over the last 24 months.
Looking at the trends over the last 10 quarters (only eight shown) simply re-inforces the obvious: Don't look at such numbers on a quarterly basis.
Comparing calendar '07 to '06, Linux-based server revenue picked up a point of market share and Windows-based server revenue picked up more than 2 points vs the combination of UNIX-based and "other-based" (which is primarily IBM (IBM) mainframe-based). That trend has continued into '08. Looking at the numbers on a trailing 12-month basis, Linux has picked up almost another point and Windows gained .8 percent of share.
This isn't really an interesting horse race any more but I feel obligated to calculate it simply because I have been doing it each quarter.
IDC also has unit data if desired.