I'm sure its baked into the simple machines code - handheld devices tend to have weird browsers, so just check the UA string, and find something like:
X-user-agent: gammas-buttberry-that-dontlike-mobi.v12345
and decide that its a buttbery, and serve up a simplified version. Actually I have no idea I never looked at the code, but people have been doing this kind of UA sniffing since the early nineties. It's useful for faking out search engines, providing a false sense of security for your scripts, etc.
