What about servers shipped with no OS?
Unfortunately, these surveys where only hardware shipped with a pre-installed OS are consdered do leave out a *huge* chunk of the servers sold (i.e. ones with no OS, which you can get from all the major OEMs).
I'm certain that a lot of Linux server installs are done on server hardware that didn't ship with any OS, but these machines just fall down the cracks - we can "guess" that they will mostly have a non-Windows OS on them (mainly because OEM Windows is cheaper than retail Windows, even in the server market), but we have no idea what flavours of OS are actually installed on them.
It's a shame these surveys don't actually include a "shipped with no OS" category - at the very least, we should know the size of that category compared to all the ones with OS'es pre-installed.