How do I whitelist hosts when using 3G? When I'm on the move I want to tether to my phone to access a few essentials (telnet, ssh, IM, a few low bandwidth websites).  What I don't want is for a load of background processes like Software Update or Google Software Updater to start downloading gigs of patches over the phone's 3G connection.  

Is there a way to set up a temporary white list of hosts to which my machine can connect and deny access to anything else?  NOTE: This must be temporary only, I obviously want full internet access when connected via a wifi/wired connection.
Little Snitch could help you out here. It's very configurable outbound firewall for Macs. The rule set it supports is comprehensive and you can block based on interface as well as IP ranges. You could set it up with everything blocked, tether, let through just the apps you want, and then enable/disable Little Snitch with this ruleset based on how you're using your Mac at any particular point in time.