I tried numerous Google static IP addresses from different data centers, and by adding "/gwt/n" to the IP address I was able to get into the Google Wireless Transcoder. In other words, you cannot block GWT (aka "Google Mobilizer") by IP address; you have to look for the user-agent.
By the way, this Mobilizer thing is bigger than I thought. While the half-dozen browsers that use Mobilizer for "compression" are all Apple apps, I spent some time looking for Android apps that did the same thing. I couldn't find any. Now I know why. My Android 2.2 smartphone out of the box, with the standard 2.2 package and not much else, requires a Gmail account and is also tied into Google Mobile Search by default. When you use Google Mobile Search you get web pages without any processing. But there is also a little "Options" drop-down box next to the links, and if you select "Mobile formatted" it uses the exact same Mobilizer engine.
In other words, the Mobilizer engine as a "compression" option in Android browser apps would be rather redundant and pointless. Google already owns that space on Android.
We can all be grateful that Mobilizer processing isn't the default when you click on a link from Google Mobile Search. It requires a couple extra clicks to use it.