The Browser Argument
Matt , we've arguing open standards , W3C compliance and , no flash , java or any other plugin requirements for basic functionality for over a decade now. Those people just don't listen .. they only know IE and their customers / bosses only know IE.
Telling them there is a whole other world out there that is not running on windows doesn't matter for them as they don't see revenue from them. (Chicken, Egg)
The argument that does work however is :
"Hey I just tried looking at your site while waiting for this meeting and it didn't work, does it work on your phone ?"
People will start to care about not being able to have basic functionality from their phone.