Hadn't found any way of finding out the UpgradeCode from an installed application, before seeing Yan Sklyarenko's workaround (currently) above. But if you/anyone else would find a way of finding out (at least) both UpgradeCode and ProductCode from a MSI, read on.
From http://www.dwarfsoft.com/blog/2010/06/22/msi-package-code-fun/, modified to allow (when launched with wscript.exe) one popup box of info per MSI (Trunicated at 1023 chars, due to wscript.echo limitation); able to input MSI(s) from the GUI as well as the CLI; some basic human input validation; removed debug code (' Set oDatabase) and 1 bug fix (DB.OpenView).
'Created by:   Chris Bennett
'Created Date: 22/06/2010
'Description:
'   Opens up MSI file(s) Passed as Arguments & returns ProductName, ProductCode,
'   The HKCR key created from ProductCode (a Packed GUID of ProductCode), the 
'   PackageCode and the UpgradeCode of the MSI. Much quicker than getting these
'   out of the MSI's the Manual Way.
References:
http://msdn.microsoft.com/en-us/library/aa369794%28VS.85%29.aspx
http://www.eggheadcafe.com/forumarchives/platformsdkmsi/Jan2006/post25948124.asp
if wscript.arguments.count = 0 then
  MSIs = inputbox("Enter in * delimited list of MSI's to query (Max 254 characters)", "MSI Product Details")
  MSIs = split(MSIs,"*")
else
  set MSIs = wscript.arguments
end if
set objFS = createobject("scripting.filesystemobject")
For Each MSIPath in MSIs
  if objFS.fileexists(MSIPath) then
    Set MSIDetails = EvaluateMSI(MSIPath)
    MSIDetails = MSIPath & ": " & vbcrlf & vbcrlf & "Product Name: " &_
    MSIDetails("ProductName") & vbcrlf & "Product Code: " &_
    MSIDetails("ProductCode") & vbcrlf & "Product Key : " &_
    "HKCR\Installer\Products\" & PackGUID(MSIDetails("ProductCode")) &_
    vbcrlf & "Package Code: " & MSIDetails("PackageCode") & vbcrlf &_
    "Upgrade Code: " & MSIDetails("UpgradeCode") & vbcrlf
    WScript.Echo MSIDetails
  else
    wscript.echo "Inaccessible; Non-existant; or Error in Path for:" & vbcrlf & MSIPath & vbcrlf & "... skipping"
  end if
Next
Function EvaluateMSI(MSIPath)
  On Error Resume Next
  ' create installer object
  Set oInstaller = CreateObject("WindowsInstaller.Installer")
  ' open msi in read-only mode
  Set oDatabase = oInstaller.OpenDatabase(MSIPath, 0)
  Set objDictionary = CreateObject("Scripting.Dictionary")
  ' Get Package Code from Summary Information Stream   
  Set streamobj = oDatabase.SummaryInformation(0) '0 = read only
  objDictionary("PackageCode") = streamobj.Property(9)
  ' Get Product Name from MSI Database
  Set View = oDatabase.OpenView("Select `Value` From Property WHERE `Property`='ProductName'")
  View.Execute
  Set ProductName = View.Fetch
  objDictionary("ProductName") = ProductName.StringData(1)
  ' Get Product Code from MSI Database
  Set View = oDatabase.OpenView("Select `Value` From Property WHERE `Property`='ProductCode'")
  View.Execute
  Set ProductCode = View.Fetch
  objDictionary("ProductCode") = ProductCode.StringData(1)
  ' Get Upgrade Code from MSI Database
  Set View = oDatabase.OpenView("Select `Value` From Property WHERE `Property`='UpgradeCode'")
  View.Execute
  Set UpgradeCode = View.Fetch
  objDictionary("UpgradeCode") = UpgradeCode.StringData(1)
  Set EvaluateMSI = objDictionary
  On Error Goto 0
End Function
Function PackGUID(guid)  
  PackGUID = ""  
  '*  
  Dim temp  
  temp = Mid(guid,2,Len(guid)-2)  
  Dim part  
  part = Split(temp,"-")  
  Dim pack  
  pack = ""  
  Dim i, j  
  For i = LBound(part) To UBound(part)
    Select Case i
      Case LBound(part), LBound(part)+1, LBound(part)+2
        For j = Len(part(i)) To 1 Step -1  
          pack = pack & Mid(part(i),j,1)  
        Next  
      Case Else
        For j = 1 To Len(part(i)) Step 2  
          pack = pack & Mid(part(i),j+1,1) & Mid(part(i),j,1)  
      Next  
    End Select
  Next  
  '*  
  PackGUID = pack  
End Function
If one needs to copy&paste any of the GUID's in the popup, I tend to find it easiest to use a subsequent inputbox, like inputbox "","",MSIDetails