Here's what happens:
My iPhone will always render the forum with the mobile theme. While I stick to the iPhone, the desktop will always render the desktop theme.
If I access the forum from the iPod Touch, it uses the desktop theme. I switch it to the mobile theme. Now the desktop will use the mobile theme.
I switch back to desktop theme on the desktop and now the iPod Touch will also use the desktop theme while the iPhone will sitck with the mobile one.
The iPod Touch & Desktop will fight for the themes while the iPhone just sits the and watches the fight.