For the most part it seems to work. A couple I tested didn't seem to finish the uninstall completly -ie delete the directory - and it does require local admin rights. Most of the users here (regretably from past admin) have local admin rights so I think it will work ok.
I found some code from Trend thought the premise was interesting - waiting for the directory to be deleted (could cause an infinate loop)

Create a login script in a GPO using the following command:
@ECHO OFF
IF EXIST "C:\tm_installed.txt" GOTO EXIT

\\\trend\Autopcc.exe

:LOOP
ping -n 60 127.0.0.1 > NUL
IF EXIST "%ProgramFiles%\Trend Micro\OfficeScan Client" GOTO LOOP
\\\ofcscan\Autopcc.exe
ECHO "installed" > "C:\tm_installed.txt"

:EXIT

This is what I am currently using...

;==================================================================
; Check and install if needed - New Trend Micro
;==================================================================
$File = 'C:\Program Files\Trend Micro\OfficeScan Client\ofcscan.ini'
$Version = ReadProfileString($File, 'INI_PROGRAM_VERSION_SECTION', 'PccNT_Version')
If Val($Version) < 10
'Need to upgrade!' ?
'Please wait...uninstalling old software...' ?
Shell '\\whchsrvr2\ofcscan\CmnUnins.exe'
'Installing new software...' ?
Shell '\\whchsrvr2\ofcscan\autopcc.exe'
Else
'Version 10 or higher - no upgrade needed!' ?
EndIf


any more thoughts I am still interested... esp local admin rights...

Glenn