If you're launching Software Update manually, I imagine you could instead run an AppleScript script which would launch SU, then click the Show Details button, but since I don't have access to 10.6, I can't experiment with such a script, and, due to the extremely quirky nature of GUI scripting, such a script can really only be written through copious experimentation.
Might be worth reporting it to Apple as a bug; presumably one who is interested enough in the updating process to check manually doesn't fit the profile of the naïf for whom the "take care of all of this stuff out of my sight" approach seems designed.
