Go to Start (Is it still actually called that officially in Vista, since it's now just a logo), right click on Computer, and select Properties. For me, I see at the top a section called Windows edition, and mine says
Windows Vista Business
Service Pack 1
While SP1 is not a miracle cure all for Vista (just got done reseating network cables and trading ports to see why mine was dropping 80% of the packets to the network), it is slightly better and worth getting. If you don't have it, go to Start, and type in Windows Update. I don't think SP1 is going out over automatic updates yet, but it will show as a choice if you run the updater manually.
Start/computer/properties shows meWindows Edition_______________
Windows Vista Ultimate
Copyright 2006 Microsoft Corporation All Rights Reserved
and that's it. Running Windows Update manually does not seem to offer me the option of installing SP1. It does give me the message that everything is up to date.
So, I still don't know whether I have SP1 installed.