Operating systems older than Vista:

SELECT Version FROM Win32_OperatingSystem WHERE Version < "6" 

Vista and 2008 only:

SELECT Version FROM Win32_OperatingSystem WHERE Version LIKE "6.0%" 

Only Win XP SP2:

SELECT Version FROM Win32_OperatingSystem WHERE Version = "5.1.2600"

W2K8, regardless of SP. (ie Not equal to workstation (1))

SELECT * FROM Win32_OperatingSystem WHERE Version LIKE “6.0.%” AND ProductType <> “1”

Only on Windows Server 2008 Core servers.  Here is a sample query (wrapped for readability, this should be done as a single line in the filter dialog):

SELECT OperatingSystemSKU FROM Win32_OperatingSystem WHERE OperatingSystemSKU = 12 OR OperatingSystemSKU = 39 OR OperatingSystemSKU= 14 OR OperatingSystemSKU = 41 OR OperatingSystemSKU = 13 OR OperatingSystemSKU = 40 OR OperatingSystemSKU = 29

These values map back to HEX values, which map back to:

PRODUCT_DATACENTER_SERVER_CORE
0x0000000C

PRODUCT_DATACENTER_SERVER_CORE_V
0x00000027

PRODUCT_ENTERPRISE_SERVER_CORE
0x0000000E

PRODUCT_ENTERPRISE_SERVER_CORE_V
0x00000029

PRODUCT_STANDARD_SERVER_CORE
0x0000000D

PRODUCT_STANDARD_SERVER_CORE_V
0x00000028

PRODUCT_WEB_SERVER_CORE
0x0000001D

Only on a certain day of the week:

To do this, your WQL queries (one filter per GPO that you wanted to set, remember) would be:

Select DayOfWeek from Win32_LocalTime where DayOfWeek = 1

Select DayOfWeek from Win32_LocalTime where DayOfWeek = 2

Select DayOfWeek from Win32_LocalTime where DayOfWeek = 3

One is Monday, two is Tuesday, etc.

Select only laptops (ie computers without sodimm memory:

Select * from Win32_PhysicalMemory where FormFactor != 12

Not a server:

SELECT ProductType FROM Win32_OperatingSystem WHERE ProductType = “1”

See: http://blogs.technet.com/askds/archive/2008/09/11/fun-with-wmi-filters-in-group-policy.aspx