One question, though, from the non-expert: how does this change while accessing Gmail through, say, an iPhone 3G network rather than a wifi connection? Obviously https is still an option, but is its preventative power affected at all from the change?

Sorry I wasn’t more specific; you are both correct. Both ends of the email communication have to to be using encryption for the communication to be secure along the whole path (plus the path between servers, if they are different). People have the same confusion about Blackberries — from the mobile device to the Blackberry server is encrypted, but of course if the mail is not destined for another Blackberry it will not be encrypted the rest of the way. But with Gmail, the most worrisome vulnerability is WiFi packet sniffing at the source, and using SSL solves that problem.

As another poster has pointed out, using https only protects your email as it goes from your machine to google’s server. From there on, it’s sent unencrypted to the recipients server. The chances that some hacker will be listening in on your Wifi network are much higher than that they’ll be listening to the traffic between Google’s server and the destination mail server, so it really does make sense to use https. However, it certainly doesn’t protect your email from snooping by the NSA.