I don't think they do. As far as I understand it, employees can choose Mac, Goobuntu or Windows (though windows has to be justified b/c of the higher overheard of support). Sounds reasonable to me.
From talking with Thomas before, if I have this correctly, is that Google doesn't allow offsite development. So it seems most Googlers have a desktop for development and an laptop for other things with NO code on it. I may have that wrong, but I'm sure some Googlers here can verify that or not.
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.
BTW, a company like Google enforcing software choices for employees... Meh.