I'm currently using the pre-RC1 of Windows Vista build 5536. Though I don't think the build will matter in this instance.
What happens is this; after downloading the update automatically upon trying to log in, SL tries to launch the file to update. Which works fine.
But, Vista pops up a warning asking if I want to allow this program to run. The program was named 5AB.exe. There was no indication that it was the SL update, and every indication that it was a piece of spyware or virus (random filename residing in a temp folder) so I said No. Curious as to what it was, I dug for it and found it, and *then* recognized the SL icon and ran it.
When Vista goes live, other uses will run into this issue and probabaly react as I did, which may cause some headaches for support.
Suggestion: Maybe, instead of using a random filename, you could use SecondLifeUpdater.exe or something similar.