Vista - Uninstall fails on Vista - seems not to elevate

I have been doing some msi package installation testing on Windows Vista
Beta2 with UAC enabled. I have a Windows Installer package which installs
fine. However, when I uninstall from Add/Remove Programs the uninstall fails
during InstallValidate with the following error:

"Error 1321. The Installer has insufficient privileges to modify file X"

The file in question has security which allows local Administrators full
control, but gives no rights to standard Users ie. not even Read access.

I am logged on as an Administrator with Windows UAC enabled. Hence, when I
begin the Uninstall I am asked to "elevate privileges" by Windows, which I
agree to.

I believe this means the Installation should be running with full Admin
access, just like pre-Vista, or like having UAC turned off. However, it looks
like at this point in the Uninstallation, the Installer doesn't really have
Admin access as it can't read the file.

If I turn UAC off, via the local security policy, the Uninstall completes no
problem.

Is UAC elevation working correctly here? Any ideas?

J