It's not "offsite development" that's prohibited so much as not allowing source code to be stored locally on laptop HDD's (even with full-disk encryption). You can develop remotely over SSH, NX, NFS, SSHfs, etc., you just can't have the source (or compilation artifacts) on an easily-stolen device.
Java developers that use Eclipse or IntelliJ seem to work remotely pretty well using NX or VNC to get a remote Linux desktop, and Googlers who work on open-source projects obviously have different rules for that code.
I'm actually not sure what the IOS devs do. They might have different rules since their projects are more standalone and not tied into the rest of the main Google source tree, but it might also be that they just develop on-site. You could probably be fairly successful with XCode using something like SSHfs if you're on a fast enough connection, but I don't know if anyone actually does that.
