How do I read 64-bit Registry values from VBScript running as a an msi post-installation task?

Not sure about launching the 64-bit script host version, but you should be able to access the 64-bit registry from the 32-bit script host using the WMI StdRegProv class, like this:

Const HKEY_LOCAL_MACHINE = &H80000002
sPath = ReadRegStr (HKEY_LOCAL_MACHINE, "SOFTWARE\Microsoft\ASP.NET\2.0.50727.0", "Path", 64)
WScript.Echo sPath

' Reads a REG_SZ value from the local computer's registry using WMI.
' Parameters:
'   RootKey - The registry hive (see http://msdn.microsoft.com/en-us/library/aa390788(VS.85).aspx for a list of possible values).
'   Key - The key that contains the desired value.
'   Value - The value that you want to get.
'   RegType - The registry bitness: 32 or 64.
'
Function ReadRegStr (RootKey, Key, Value, RegType)
    Dim oCtx, oLocator, oReg, oInParams, oOutParams

    Set oCtx = CreateObject("WbemScripting.SWbemNamedValueSet")
    oCtx.Add "__ProviderArchitecture", RegType

    Set oLocator = CreateObject("Wbemscripting.SWbemLocator")
    Set oReg = oLocator.ConnectServer("", "root\default", "", "", , , , oCtx).Get("StdRegProv")

    Set oInParams = oReg.Methods_("GetStringValue").InParameters
    oInParams.hDefKey = RootKey
    oInParams.sSubKeyName = Key
    oInParams.sValueName = Value

    Set oOutParams = oReg.ExecMethod_("GetStringValue", oInParams, , oCtx)

    ReadRegStr = oOutParams.sValue
End Function

NB: I'm under a 32-bit OS right now, so can't verify that this example works. Beware of bugs :-)

See also the Requesting WMI Data on a 64-bit Platform MSDN article for more info on the subject.

Using Microsoft's documented approach, Helen's answer is absolutely correct.

However according to my own tests it turns out that it is enough to specify the __ProviderArchitecture context flag only at the time the connection to the StdRegProv provider is established.
This makes things much simpler as only setting up the provider needs to be encapsulated in a separate function, otherwise one can use the regular API.

set reg64 = MakeRegLocator(64)
reg64.GetStringValue , "SOFTWARE\Microsoft\ASP.NET\2.0.50727.0", "Path", path

WScript.Echo path

' Establish a connection to the local 32 or 64 bit registry hive as requested.
' Parameters:
'   RegType - The registry bitness: 32 or 64.
function MakeRegLocator(bitness)
    set ctx = CreateObject("WbemScripting.SWbemNamedValueSet")
    ctx.Add "__ProviderArchitecture", bitness

    set locator = CreateObject("Wbemscripting.SWbemLocator")
    set services = locator.ConnectServer("", "root\default", "", "", , , , ctx)
    set reg = services.Get("StdRegProv")

    set MakeRegLocator = reg
end function